When i sit up from a prone position, i have a large abdominal bulge, like alien! ct scan shows nothing. what's up?

Diastasis Rectii: What you are describing is "classic" for separation of the rectus muscles. This is often mistaken for a hernia, which it isn't. Since it is purely cosmetic, there is no need for surgical correction. This tends to occur in men as they gain weight ("beer belly") or women after childbirth--a "tummy tuck" corrects this separation. Btw-this can be seen on your ct, but one has to look for it.

Diastasis: If the bulge is in the middle of the abdominal wall between the belly button and the bottom of the breast bone, it is known as rectus diastasis. The thick band of scar tissue holding the "six pack" muscles together is thinned out, which is why it bulges duog a sit up. It's normal, not dangerous, and doesn't require treatment.