Hi Mar-
Are you doing extremely intense ab work, that you aren't customarily used to doing?
I have found that when I have worked a body part really really intensely, the next couple of days, it gets kind of swollen. I really notice this in my legs, and in fact, I will actually gain an extra pound or so, from water retention after a particularly grueling session.
I had since found out that when you are REALLY working your muscles, you are actually kind of damaging them, which results in the swelling. When your muscles repair themselves, that is what is giving them the growth/strength gains.The bloat/swelling goes away. What is left is increased muscle gain.
If you think this might be what is going on, I would suggest giving yourself a couple of rest days in between your ab workouts and see if the bloating minimizes.
Good luck and I hope you feel better.
Lynn