Help......

Fefe
I constantly feeling the baby stiffen up in my lower stomach. Not sure how to describe it. Each time she loosen up i have to pee. I am 31 weeks. This is bothering me unsure if I should go to the doctor or ER. It has been happen alot lately. If am laying down i have to change sides, if am walking i have to stop until tension goes away. What would this called?