That sounds like egg-holding to me. Get some nice, warm water, olive oil, and a towel. You may want plastic gloves. What I did when our hen had this was I soaked her a little in the water, maybe with some Epsom salt. Massage her rear, maybe even let her walk around a little. The main thing is to help her RELAX. It seems that she has a broken or partially formed egg inside her; she needs to get that out. If there is a shell, it complicates matters and makes it even more serious. Use olive oil on your finger and around her vent to try to get some of the white, yolk, and/or shell out. You may have to stick you finger inside her vent, too. There are two "openings" inside. I think the reproductive opening is the top one. She is in pain, so be very gentle and calm.
I hope someone who knows more about this can help. I hope your girl's OK!