it could be round ligament pain, i was around 16 weeks to when i noticed the same thing at night, i looked it up and from what i could tell it was. all of your muscles and everything are adjusting to your growing uterus. like Joy said your already doing whats seemed to help me, but if you roll over and stuff in your sleep it can still hurt.
Yeah, I have been the worst person to sleep with. My hubby says he feels like hes been in a fight with me everynight. I toss and turn and throw my 7 pillows on top of him. I have always been a mover and a shaker when sleeping. I have a feeling the pain is going to get worse i cant seen to lay still. Sometimes Hubby will roll me around making a pillow baracade around me so i stop moving.
Do you move around a lot in your sleep? There are so many discomforts in pregnancy. It sounds like you're already trying to do what you need to do which is lay on your side with a pillow between your knees. You could also put a pillow under your hip and behind your back. It's so easy to pull those taut muscles and cause pain in your belly.