L.S.
Perhaps your feeling the rectus muscles of the abdomen separating. (These are the long vertical abdominal muscles.) I used to teach a pregnancy fitness class, and we used to teach women to feel for the muscle separation. (We'd adapt some exercises if muscles were separated.) If you lie on your back and pick your head up slightly off the floor, sort of in crunch fashion, and feel your abdomen with your fingertips just below your belly button, you should feel a groove running vertically from the belly button downward. If that groove is three fingers width or wider, the muscles have separated. For some women, this happens with the first pregnancy, in later pregnancies for some, and for some it never happens.
Then again, maybe you're having twins! :)
Good luck and congrats!