Dirty vent, lethargoc, sitting back and weak(ish)

Marjory, Light Sussex, about two years old - has always been a wheezy bird but a good layer and a lively girl. Today I noticed she seemed lethargic and was last out of the hen house this morning.  Checked her over and noticed her vet area was really grubby, so gave her a bit of a wash and a quick spray of iodine (as it looked like she has been pecked in the past).  Her comb is bright, she's eating and drinking but now seems to have difficulty walking and is sitting back.  The other girls are fine.

Any suggestions what might be wrong?

when did she last lay an egg?

She may have an egg stuck inside her in which case her muscles will need relaxing - pop her in a sink of warm water for 30 - 40 mins (keep the water warm), this should relax her muscles to pass an egg - then towel dry her off and finally blow dry her with a hair dryer on a warm setting.
She could have had an egg break inside in which case she will need a trip to the vets for some antibiotics. If an egg has broken the vet may need to remove any remainder from inside her or.........
She may be getting ready to lay a soft shelled egg - these do make them miserable but once they have passed the egg they generally perk up after a day or so.

I have just had the same problem with my white leghorn hen could see she was uncomfortable so took the advise of putting her in warm water and it worked she layed a soft egg. It took he about 20 mins to recover from it and she is running about happy now. It does worry you when i bathed her she layed on my knee and was going to sleep then she got up and layed bless her. Thanks for advise on this forum I would be lost without it. Hope your chook is fine too.