Waking up at night with a round belly

I'm 17 weeks and am barely showing, if you even call it that. I sleep on my sides and back and wake up 4-5 times at night due to having to pee or just being unable to slee and everytime I wake up, roll over on my back and rub my belly (out of habit). The last 3 or 4 nights I've noticed it feels like a volleyball under my skin. It's so tight and round and I've even felt movement. But when I stand up to go to the bathroom it slowly goes away but sure enough to 3-4 hours later when I wake up again it's back and goes away! It's crazy! Anyone else do this? Or know why?