What Causes Small Red Bump On Breast?

I have a small red bump on my left breast. I'm about to turn 21. It has been there for a few months now. At first it seemed as if it was a pimple, but it hasn't gone away. I was taking a shower just now and noticed that the hot water made it redder. It usually looks like a bruise, but now it looks more like a hickey. I'm getting worried that it's cancer...