A cpl of days ago I saw a small pimple like bump on my breast. I popped it & pus came out. It's now red, swollen & larger. Could it be breast cancer?

Unlikely. The adage "common things commonly happen" is true. There are sweat & hair pores on your breast & these can be invaded by skin germs and form an pustule or abscess just like they would on other body surfaces.Healing is promoted by moist heat(improves blood flow) or opening & draining them. Sometimes they require an antibiotic. It is worth a visit but I wouldn't freak about cancer as my first thoght.