Inguinal hernia. Never worked with an ab machine before, but when you do anything that increases the pressure in your abdominal area there is that TINY risk, especially if you had surgeries or operations in that area. The scar that is left after the healing process is not as strong as the original fascia, and it tears and your intestines literally are pushed out through it under your skin. Its not an emergency, but if you notice it getting bigger, you might want to lay off the ab workout for a while. Is the area fluctulant? does it go up and down like a balloon?
I think you have nothing to worry about. Maybe those once flabby ab muscles just got residual tension in them, and thats why they look so much more defined now (they are basically still flexing).
P.S If anyone has a problem similar to this, and they noticed ab pain (I am talking about ab pain that makes you want kick your poodle) , diarrhea, huge bloating. GET YOUR ASS TO AN ER, that's what a strangled hernia feels like.