What Causes A Reddish Lump In Between The Breast?

A couple months ago I saw a bump in between my breast. I thought that it could be from not scrubing hard enought in the shower or from my sports bras so I left it alone and it went away. But know its come back and its bigger and hurts when adding pressure. It looks like a bruise and is red. What do you think it is? Should I be worried?