I get BH when baby changes position (as well as late in my pregnancies when my uterus is toning for labor, when I'm dehydrated, etc). Maybe you've got a combination thing going...baby can surely spin and flip around in there with so much room to play right now. Maybe your uterus is just sensitive to movement?
Another thing that comes to mind is are you getting enough calcium/magnesium, because deficiencies in those can cause muscle cramping.
Are you drinking herbal tea with RRL? Sometimes my body is more sensitive to herb infusions, and I remember with my second baby, RRL tea brought on CRAZY contractions one night.
That sounds awfully uncomfortable, whatever the cause. I hope you find some resolution soon!
Mommy to ds (4.5), dd(3) and growing a little Valentine's Day babe!