I have the same thing, even touching my tummy hurts, but i was underweight when i concieved and has gained oved 20 pounds so far and im only 5 months along. My dr said im gaining weight so fast is my bodys way of taking care of me and the baby. Idk, but i think the pain if from the stretching. I brought it up to my ob who is also my midwife and she said it was normal so i stopped stressing a bad. She also informed me if u do stress alot the muscles in and around ur belly can tighten and that can make ur tummy sore... im sure everythings fine. Goodluck!