Tons of braxton hicks 21 weeks

I've had at least 6 in the last hour. I haven't been timing them but I finally sat down to give my body a break. I've drank at least 50oz of water today. I had a bunch of BHs with my first baby but I slightly worried how strong and often they are with this one. My OB tells me I have an irritable uterus. Baby kicks and moves between the contractions so I know she is ok, but when do I worry?