I feel that all the time!!! I am 39.5 wks now and I still feel it. I also was concerned when I first felt it and I still wonder what the heck he's doing in there! Everything is fine with the baby though so it's just something funky he's doing in there!
I did ask the OB about this and he said any movement is good. An active babe means a healthy babe!
Check it out with your Doc/midwife if you are concerned but if the h/b is strong and healthy then it's probably nothing!
I also didn't feel this with my first. Must be a second baby thing!