Lacey's Mummy, at my group antenatal physio class they were talking about separation of the muscles in some women! Basically the big long abdominal muscles, the rectus abdomini (the ones that give men a six pack) normally only spread or thin out a bit later on when bump is big, but in some women this can happen early on and it gives pain! The physio said you can check (I've just copied this out of information she gave us!)
1. Lie on your back with your knees bent and your feet flat on the floor. 2. Slowly raise your head and shoulders off the ground. This should cause your abdominal muscles to tighten. 3.Place your index and middle fingers just below your belly button. 4.Press into your abdomen with your fingers. You should feel a soft gap between two hard muscles. 5.Measure the space of the gap using your fingers. If the gap is greater than two finger widths, you may be suffering from separated muscles.I think the proper name for complete separation is diastis recti.
Or maybe just bump growing quick??