sacroilliac. I had trouble with mine in Track & Field in my college days. Unfortunately, I never really found anything that made it feel better. Rest usually made it go away and then I could start up again and pray that it didn't come back... for the most part it didn't. I really only had pain there on 2 or 3 occasions over 5 years. Biking has never given me any trouble.

Hopefully you have a good PT and they can help you out!