Have little bumps on my arm that have been there for months. don't go away. expand when scratch, and go back small after a while. what is it?

Hard to know: .. without performing an examination. Some potential diagnoses involving persistent bumps that worsen with scratching and that are otherwise stable include prurigo nodularis, lichen planus, or papular urticaria. A dermatology consult may be necessary. A good starting point is to avoid scratching at all costs and keep skin well moisturized (can start with aquaphor or vanicream).