They could be cancer lumbs... I would see a vet just to see what they are
They could also be sweat bumps. Horses get them when a sweaty piece of tack is not cleaned properly or if the horse is sweaty and not towelled off/washed properly.
How to treat depends on what they are. They could be fly bites, they could be food allergies, they could be rain scald, they could be nodules (nodular collagenolytic granuloma), it could even be the start of mange. If you're worried, I'd get the vet. Washing with hibiscrub might help if they start to irritate her