She could have an egg stuck - see above suggestions, and 'search' on 'eggbound' - however, I have to warn you that what you describe is also the classic presentation of internal laying, where the eggs get directed into the abdominal cavity instead of 'down and out' the usual way. Internal laying is only really correctible with Expensive surgery, and my understanding is that it is almost always fatal sooner or later (I lost a hen this way last fall, a month or two between first symptoms and passing).
Hopefully it is jsut a stuck egg that will resolve itself.
Good luck and fingers crossed,
Pat
