Any idea what this might be?

[sheep73]

Our gorgeous 8 year old Welsh Section A lives out with two friends. She is half clipped.
Over the last couple of weeks we notice some patches of hair loss where you can see her dark skin underneath. It's well behind the girth sort of on the side of her tummy.
She has it on both sides.
It doesn't seem to bother her and she's eating like a train.
A friend of ours suggested sweat patches due to over rugging so we're leaving her rug free now in case she's getting too warm and damp.
Any thoughts?

Looks like rain scald to me (a mild bacterial skin infection), more common to see it on the hindquarters or backs of thin-skinned horses that have been living out without rugs but I can see the rationale that it might have been caused by sweaty/damp and humid conditions under a rug as anything bacterial loves a nice warm moist environment to grow in. If it is that, the treatment is just washing with dilute hibiscrub or another gentle anti-bacterial solution then keep it dry and it usually clears up within a week or two (I think you can also get a cream from the vets) - trouble I guess is you don't want to rug her if the suspicion is she's getting sweaty but you probably don't want her getting soaked in the rain or cold either, if the weather does turn have you got a very light breathable sheet you can pop on?

Mine has a couple of similar looking patches - his were caused by hives which suddenly came up one day a couple of weeks ago when the temperature during the day suddenly rose and he got a bit too hot. When they went down they scabbed over and now the scabs have gone we’re just waiting for the hair to grow back.

Difficult to see but could be shedding the winter coat. My lot (in Portugal so a few weeks ahead) look very moth eaten and tatty!

Thankyou all that's helpful. I think we'll try hibiscrub this afternoon and keep her as rug free as possible and hope the weather stays dry. If it gets wet we'll bring her in.

Farrier has concurred it's some kind of sweat / mud rash.

many thanks all for your help!