Your bub could be trying to move around - Flynn did this at about 35 weeks. Silly bugger tried to turn breech and I was gasping with pain for about 30 minutes, thinking I was in labour. Apparently bubs try this occasionally, but by 30 odd weeks are often too big to manage to flip themselves so get stuck and scramble around.
Like others have said, it could be BH - some women get them quite strong. Go to your OB if you get really worried. Though all sorts of wierd aches and pains are normal