Tummy tightening?

Katie • Anya Rose Nishioka 6/8/2016❤️🌸 Young mommy (20 years old)👶🏻 instagram is katnishioka.fitness
I'm 19 weeks and recently I've noticed that at night my belly will tighten to be almost rock hard and then within 45 seconds it loosens again. Has this happened to anyone? Any idea what it is? I'm going to ask my doctor about it because my appointment is Monday but any personal experience with this?