I'm almost 30 weeks pregnant and for the past 30 min my uterus has been rock hard from top to…

I'm almost 30 weeks pregnant and for the past 30 min my uterus has been rock hard from top to bottom. I don't think it's contractions because I've had BH and that comes and goes. This has been rock hard for 30 min straight and is a little painful and definitely uncomfortable. Any advice? What could it be?