I'm thinking since she is a broody and has been sitting this would be the reason for the missing feathers and possibly the redness/irritation.
Check her for any signs of lice/mites and consider changing her bedding if you haven't done so. Hopefully since she is up and moving around with her babies, it will start to heal. You can try putting a little ointment like A+D on the irritated spots (if any spots looks like it's broken skin use vetericyn or plain neosporin).
As far as yeast infections go, I suppose they could get them on the skin as well, but normally yeast/thrush infection would be at the vent. It's call Vent Gleet. Information and treatment is in the following links.
I agree with Wyorp Rock - when hens hatch eggs they tend to pluck out / lose their belly feathers, as it means their skin is in direct contact with the eggs to keep them as warm as possible. If she is not irritated by the feather loss and the skin doesn't look infected then I would leave things be.
However it might just be worth checking for mites / lice - my broodies tend to get infestations towards the last week of brooding - being on the nest 24/7 they are just a 'sitting duck' (pardon the pun) for parasites. If she has mites or lice and you pick her up or put your hand under her wing or belly for a couple of minutes you will soon find out, because they will move onto your skin too! If that happens then you can dust both her and the chicks with a pyrethrin-based powder.
Started out with 3 birds. Currently at 13 pullets and 2 roos
- chicken maths is definitely getting the better of me!
Member of the Derperella Club - we're all just going round the rooster here.
RIP Blackie (the best hen ever), Rusty (too curious once too often) and Cinders (my grey girl)