I have a big pimple at the side of my nipple. Could that be a tumor?

--Jess

This "pimple" could be just that -- an infected gland or duct. It could also be a prominent Montgomery's tubercle, a normal part of the breast. It could be many things.

You did not mention your age, which is usually a factor in the diagnosis of most conditions. The best thing for you to do is to see your doctor and find out exactly what this is. This is a case where a description over the Internet cannot give anywhere near enough information.