I've had a painless pimple like bump on the side of my left breast for about 2 weeks now, what could it be?

Most likely nothing: most likely it is something benign. just like other skin covering your entire body, you can have the same problem on the skin over your breast. you can have clogged pore, or infection in the root of the hair, or other kind of infection/inflammation. if you are worried about it so much, go to see your PCP and get yourself checked.