Ouch!

Did it cause her any discomfort?

I guess my only two ideas is that it might be something like fibroids that would build up tissue, or it might be something to do with the bone in her pelvic area... It might have been something that she was unaware of until you had a problem.

I guess regardless of whether or not it is a common problem, it's still a problem, at least for any partner of hers, so I guess I'd advise her to see a gynae... pleasant though that is...