Please don't panic. The most common causes of pelvic fluid are normal. In fact, it's usually referred to as "physiologic fluid," which means it's just a normal part of how your body functions. The lining of the abdominal cavity is always secreting some fluid to keep things lubricated. Some of that can accumulate in the lowermost part of your pelvis around your uterus and rectum (pelvic cul de sac). The amount of fluid there increases around the time you ovulate. It can also be due to an ovarian cyst rupturing. It's not likely without other symptoms or findings, but it can be related to infections like appendicitis or diverticulitis. It would be exceptionally unlikely to be related to cancer without any other abnormal findings.
Go get the second opinion, but I suspect the doc you see will be puzzled as to why your first doctor made a big deal out of it.