Rare condition that causes the center of my abdomen to puff up or bulge. It is about the size of a hot dog bun. No pain. Happens during crunches?

Diastasis Rectii. The rectus abdominis ("6-pack")muscles are normally fused in the midline. If they separate from one another, a tent-shaped protrusion is evident in the midline when contracting these muscles. The strong (but thin) tissue that envelopes these muscles, called fascia, remains intact; therefore, there is no hole (like a hernia) and surgery is not necessary. Great description!
Diastasis recti. It is medically called diastasis recti or simply a type ventral hernia resulting from separation of your rectus muscles (one mucle band on each side). This is mostly a cosmetic issue and not medical--as it almost never causes cause pain. If it is bothering you--cosmetically, consult your doc. A tummy tuck type of procedure can be done to take care of your "hot-dog-bun" problem...Good luck.