Belly really tight & irritable...what does this mean?

I'll be 24 weeks (6mos) on friday and since yesterday my belly has been feeling really tight and irritable. I've been drinking water and trying to stay hydrated but it still feels the same. I dont know why its been felling like this, could I be dehydrated and should call my doctor. I've heard that when you are it irritates your uterus or what else could be causing this???