What Causes Red Colored Rash On Chest After Finishing Antibiotics For Tonsillitis?

I have just finished up taking antibiotics that I have been on for a month for tonsillitis. I now have a red spotty rash all up my chest. A lot of them look like tiny pimples. It burns just slightly but is uncomfortable because there are so many. What could this be ? What can I do to get rid of it? Thanx