This could be caused by something you ate and are having an allergic reaction. Or something you touched and are having allergy symptoms. You need a cream called hydra-cortisone creme .5 of 1% or 1% by prescription rub this on to eliminate itch and bumps. Much better relief than calamine lotion.
This is impossible to answer without seeing it. You need to consider how quickly the bumps have developed, are they flat topped, what size are they, do they have any colour change eg purplish or white lines through them? The best starting point is have you changed your washing powder or come into contact with anything you aren't used to? If not, consider seeing your doctor. Best to be safe than sorry as my mun would say.
Red small lumps on my lower arm which itch
I have a lot of little spots with 1 or 2 big spots that don't itch but just on one arm