Um, this is possibly totally inappropriate fearmongering sounding, and I apologize if it sounds all dire and stuff, but early in this pregnancy I had pain in my left breast close to my pit that turned out to be a benign tumor.
At that time, I learned that pregnancy can (for certain age groups, like mine) increase the chance of certain types of growths. Soooo...after that experience, I'd probably suggest getting it checked out by your provider ASAP.
And I apologize (again) for providing such a bad, worst-case answer. It might be something so minor like a too-tight bra backing up your ducts.