sick chicken, help please

she has lost all the feathers around her vent, and has very messy poo, under her vent looks to me like its bloated. i have isolated her from the other chicks because i think they have taken a liking to the few dirty feathers left around her rear end.
apart from that she is acting normally/ not loosing weight, but she is laying a very strange wrinkled egg.
she has only been like this since the weekend, but she always seem to have a messy vent
any help would be much appreciated