What Causes Pimples On Breast With Puss Suggest?

I got what appeared to be a pimple under my breast that popped up all of a sudden after taking a nap one afternoon. I work out a lot and thought it was a pimple from sweating. 3 days later and I was going to take a shower and was examining it and noticed and white head so I squeezed and a large amount of a milky white puss came out that had a foul smell. After that followed pain around that area into my armpit and some nausea. It is right where my underwire hits my ribs.