I am constantly having that feeling too, sometiimes it really hurts as my whole tummy feels tight and round like a coconut. And most of the times when I feel it, I can feel my baby's head with my hand. Maybe it is the baby turning around and head is on the front. I am now 32 weeks. I had this with my second baby too.
It could be Braxton Hicks contractions...I felt them when I was 23 weeks. Every time I felt the baby move around a lot my stomach started tighening. It's totally normal. My doctor says if you feel BH contractions more than 6 times an hour before 35 weeks then call. But after 35 weeks it's normal.
I get this all the time. I asked my midwife about it and she said it was more than likely the baby moving. I also get contractions when this happens with me but I am also 36 weeks.