So scary! I just (half an hour ago) happened to read a magazine article in Parenting Magazine that covered this exact thing. Good news - she is probably fine!!! Here is what the article said:
6. Your 2-year-old topples out of the shopping cart. The goose egg is huge and he's hysterical. Head to the ER?
Probably not -- screaming is a perfectly healthy reaction. "The vast majority of kids with head injuries are fine, and the size of the bump rarely has anything to do with the extent of the injury," says Dr. Zibners. There are loads of blood vessels in the head and face, so the swelling can be dramatic. The exception: Any child under a year of age who experiences head trauma should always be checked out by a physician because signs of injury are harder to detect in a young infant, she emphasizes.
For kids older than 12 months, you can take a watch-and-wait approach. "If your child is crying but can get up on his own and is moving about, he's probably just fine," notes Dr. Zibners. Keep an eye on him for the next few hours to make sure he doesn't limp or favor one arm; vomit, especially after some time has passed; or become sleepy (and it's not his usual naptime) or especially irritable. If you see any of these signs, call the doctor. And, of course, if your child is motionless or unconscious, or refuses to move after his fall, you should call an ambulance right away.
I hope she is ok. I know how you must be feeling tonight - please try not to feel so guilty!