What Causes Lump On Lower Part Of Right Breast?

I have a lump on the lower part of my right breast above my ribcage. I noticed it recently and it's really starting to freak me out. It doesn't hurt, but I'm not sure if it could be something to be concerned about. Or if it's just a work related injury. I work for a shipping company and the only incident I could relate this to is carrying a few boxes against my chest down a flight of stairs.