What kind of rash is it?
Infected hair follicles (called folliculitis) are often a problem under the arms. If the bumps seem to each surround an individual hair and/or pore and are slightly raised, you should see the doctor. He/she will give you a mild antibiotic if it is folliculitis, and the rash should go away.
If you think deoderants are causing your rash, discontinue using them and see what happens. If your rash disappears, you will have to search for alternative ways to keep yourself fresh-smelling (you can ask for ideas on this board!).
If you stop using deoderants, and the rash remains, you should probably see a doctor. Your family doctor can probably help you out.
Good luck!