I had this too :( I thought a UTI or pulled muscles. My doctor said neither, that my daughter had a growth spurt causing a little bit if bruising (especially where she pokes her butt out the most) and it was just my ligaments and muscles stretching. Now at 36 weeks its not as bad, but 27 to about 34 were the worst for my sore belly
If it's tightening it could be Braxton hicks contractions.. if not then you should call your doctor or midwife.