Prophet hit the nail on the head with the sweating topic.
How old is this dog?
Younger puppies, often get, what's very similar to human's acne. Usually as they get older the problem disappears.
Mosquito bites is a possibilty, although her arm pits doesn't seem to be a normal place! LOL
How long have these bumps been there?
I'd perhaps wait a few days, if they don't go away, a trip to the vets would be in order, even though it doesn't seem to be bothering her.