I'm new here but have been studying my "Storey's Guide to Raising Chickens" profusely and this sounds to me like she has a prolapsed uterus. This occurs when a hen is too young to begin laying but does, or an extra large egg gets stuck in her oviduct. If this just occurred, you may be able to reverse the problem by using a hemorrhoidal cream; think Preparation H. You can also try gently pushing the protruding tissue back inside. Either way, isolate her until she heals. If you leave her exposed to the other birds they may peck at her prolapse and this can lead to hemorrhage and shock.
If the hen is mature, prolapse can occur when obesity is a problem.
Hopefully this is of some help to you. Again, I'm no expert but this definitely sounds like the problem. Best of luck; hope your girl heals.
UPDATE: You may want to post in the Emergencies/Diseases thread ASAP. You may get more info there.