Could be fungus. Use fungus spray
My mare has this between her hind legs too! I think they're insect bites or something..😬
It's hard to answer you without seeing the bumps. Maybe is it an insect bite ? You should wait a couple of days before deciding anything, it may just be an allergic reaction.
However if the bumps are still her in a few days, I advise you to call a vet.
Good luck :)