A question I often get late summer is “my horse has lumps on its back has the saddle caused it ?”
These can often be caused by insect bite as they like the top of the back area best
This can be compounded if you have kept a sheet on regularly and then decide to give them a day in the sun ( they are considered very tasty)
Treat with a mild cream and give your horse a few days off riding, keep the sheet on and use flyspray if needed.
Check with your vet if anything gets worse.