Poorly chicken

Morning all.  I seem to have a poorly chicken.   She has been off it for a couple of days.

She has a dirty vent although there seems to be no trace of diarhea (sp) in the house or run.  She was eating yesterday but I haven't seen her eat yet this morning.  She seems to be just sort of waddling around and then not exactly squatting, but standing with her legs open looking quite uncomfortable.  She also seems to be making a squeaking noise rather than a cluck.

I picked her up this morning (she isn't the easiest of chickens to catch) and she feels really quite heavy compared to the other chicken.  I couldn't see anything physically wrong with her but when I picked her up her little beak kept opening and closing as though she was trying to say something but nothing would come out.

I don't think she has been laying eggs for some time now (probably a couple of months) although I am not 100% sure on that.  We now only have two chickens (Mr Fox got the other two about 3 months ago) and I only ever get one egg a day now.

Any advice would be most appreciated.