Well it could be that you have a cracked rib which they won't be able to determine until after the baby is born, contractions or quite simply that the baby is kicking or moving!
Contractions are easy to rule out- if they are not consistant, don't increase and don't get stronger!
I am almost 37 weeks and I used to be like super skinny so at this stage of my pregnancy my skin is kinda sore from all the stretching and it hurts more when she moves and it is at the top of my uterus because she is head down! So more than likely your baby is head down and just stretching its legs!
Keep us posted!
Good luck and best wishes
-jessika-