With my first i had terrible pelvic pain at about 28-30 weeks and then for the rest of the pregnancy as well. Once i had delivered the paun persisted and made it very painful to walk and run. I got my hips put back in place and it was instant relief. A prenatal massage therapist can do that. I am 17 + 4 wks now and have had the same pain again since about wk 14. It feels like my pelvis is breaking! Time to go back to the MT!
It's good that you are following up with your OB tomorrow. As baby grows, it can put more strain on your pevlic area. You should talk to your doctor about using a maternity belt for support. It can help relieve some pressure. Also, look into prenatal yoga, they teach you gentle exercises to relieve the pain and pressure.
I was noticing the same thing (I'm 24+3 with #2) in my pelvis, dead center, and in both hips. I think it's the joints relaxing and getting ready to spread for childbirth.
If it's bad tell your Dr and he can let you know what to take for relief...