Every woman is different and every pregnancy is different! Sorry, I know that's not helpful. They shouldn't be 'painful' but they can certainly be uncomfortable, particularly when baby is in an odd position. Usually it's the top/front of belly that tightens and if there's pressure/pain radiating to or from your back and/or pelvis def get checked out.
Towards then end of all of my pregnancies my BH get downright painful and pretty consistent throughout the day and sometimes the night as well. Get yourself checked out if there's any other signs of labor or if you're worried. Better safe than sorry.