These bumps could be caused by different things such as cysts, abcesses, warts, and hematomas. The way to determine what they are is to take your cat to the veterinarian.
Could be anything from flea bites, mosquito bites. Cats can get rashes from food and outdoor allergies. Put some hydrocortosone cream on them if they do not heal up then I would suggest calling your local vet and getting your cat in to see them right away to rule out mange.
It could be a number of things, including a reaction to something as common as fleas. The most important thing would be to take your cat to the vet to be sure that your pet is not in any pain.