I just picked up a couple nice sized Dats about an hour ago. They were fine in the store but when I saw the one after I had put him in the tank, it looks like it popped out a hemroid or something. What the hell is up with that??
I have seen and heard it happen from super heavy feeding. The above advice about clean water and small amount of salt and it should go back in. I would do an immediate water change even if your water is great. I'm not sure why or how but it works. Obviously cut feeding down a bit.