I understand it's tearing you up to go through this with your daughter. Before you try to dismiss it like your doctor says, here's what I'd do. With all due respect to doctors, sometimes there IS something going on, and information is power, so lets get some information so we can decide if any source of terror is out there that we as a parent aren't fully aware of:
She started again last week. Go back two weeks in your calendar and write down ALL the things that influenced your daughter then and routine activities, who she watches, plays with, new neighborhood playmate like a grandchild visitin down the street, day care, babysitters, visitors (adult and children) TV shows, videos, everything.
I mean really write down, because it's a long list, and your daughter's terror is possibly in that list. I'll bet the reason she's replaying something in her head that is terrifying was inspired by something she saw or experienced from a trusted source around her. Heck, she may have seen something out the window happen at the neighbor's house or a dog catch a bunny, who knows?
Look at the list, and while you are thinking through the list, observe your daughter through this week, and see what she looks at and pays attention to (the family dog, the fish tank, etc) and add those to the list, see if any of those were there one to three weeks ago.
Now see if any of those sources had a major change in it (babysitter gets a new boyfriend who might stop by while you are gone, holiday guests who got too in-her-face-give-me-a-kiss invasive with her while holding her and passing her around, etc). And nonchelantly ask a few questions - anything happen unusual when you were babysitting here? (washer overloaded and pounded against the wall sounding like hell breaking loose?)
I know it takes discpline and concentration to make that list, however once you have that list, look at it and and see what might have put some violence into your daughter's world. If she's still doing the things you wrote down, notice when she's doin it, and while it's goin on, I'd ask your daughter a couple questions about how she likes that show, if she is in the midst of watching a violent TV show, video, or children fighting at the playground, or adults fighting. She'll tell you if it's scarey; I'd recommend you not fish for "is it scarey?" info, or you'll possibly train her to manipulate your attention. "how do you like it? or "what do you like most about it?" are non-directive questions when she is engaged in something that you think may be exposin her ot fear or violence"
Has she been left with an adult or couple recently when you were not around who might be going through holiday tensions or finanical issues (or simply abandoning her to isolation while they focus on their lives while a big dog in the family knocks her down and slobbers on her)? An adult fight about finances or inlaw behavior can turn ugly, and a child who witnesses doesn't care about the content of the messages or understand about how it's not about her personallly, she only understands the ugliness and unstable/emotionally unsafe adult behavior.
You get the kind of things that can scare kids that she might be exposed to by people who dont watch her surroundings.
I'd also go right over to the calendar and put on the calendar when the terrors occurred, and when they occur again if they do.
When they occur again, while she is engaged with you for comfort, ask her to point to what is scarey so you can take care of it. She may not be able to do this at first, but if she's a cooperative kid, she'll be pointing when her mind connects that you have asked her to help you understand what's scarey. Whatever you do, DONT tell her that her pointing isn't scarey. DO ASK - give her the benefit of the doubt and calmly like an adult to adult, not emotional, just ASK her to tell you what it does that is scarey.
She'll tell you eventually, because she trusts you are helping her. The danger is that if you coddle her and dote on her fears, it may encourage her to amplify her terrors to get more attention once she has processed the terror. So treat it like a spot on her shoe, just ask where it came from, no emotional overtones of overly concerned adult, I know that will be hard, but a matter of fact approach will tell her that terror is a matter of fact thing that adults can talk about, not a emotional chain she can pull on every time she is needy.
Be prepared for her to have beeen scared by a person, and simply allow her to live her life without being near that person (unless there's your supervision). She'll likely demonstate avoidance or ambivalance or fear toward that person or the person she perceives to be in charge when the problem happens (who didnt' protect her).
Some kids have a frightening overbearing (or worse) aunt or uncle who gets overly involved and is outright scarey to a small kid, especially if you are giving her a healthy environemnt so she is trusting of family. It could be nothing other than that, or it could be worse. Regardless, a systematic discipined list of things is where you start, and DON'T LEAD HER. if you lead her with quesitons "is this scarey?" type quesitons, she'll follow to please you. Let her lead you to eliminate things on the list, with non-directive questions "hwo do you like it" or explore it matter of factly; you're an adult, show her problem solving is a thoughtful focused acctivity, not a dramatic or terrifying thing or one that plays into her lead of intense emotion, especiallly if it happens again.
I wish you the best, and I hope it is nothing like your doctor said. However let's get the facts before eliminating your daughter's honesty and perceptions.
Peace and Joy this wonderful season.