OK. I recently discovered that I have a big bump under my right brest and it hurts like crazy... Whay might it be... HELP

We can't see it, poke it, biopsy it, lance it, or surgically remove it. Call your doctor and make an appointment.

It may just be fat it finds some weird places to go

Let your doctor know about it and that you are worried. If you don't have a doctor find a clinic and schedule and appointment. It should be examined. It could be something bad like cancer.