It sounded like a tailbone bruise to me too -- the coccyx. If it hurts to sit and especially if it is excruciating to go from sitting to standing...
I had that happen when I fell down the stairs a number of years ago. In fact, it was one of the motivators to finally lose weight! Apparently lots of women have that happen in childbirth. If that is what it is, there's not much they can do.
However, keeping in touch with the doctor is the best thing, as diagnoses via internet tend to be unreliable!
