Did you do a lot of activity the day before? Once, I did a lot of walking and sorting and I woke up and my muscles in my stomach hurt. My little guy kicked away but if I moved around, I would feel sore. I would say if you don't think it's from a way you slept, moved during the night or day before, it can't hurt to call your doctor. If it's just sore but you're baby is moving and active, I'd just keep an eye on the soreness.