From an optimistic standpoint, and also to the best of my knowledge, cancerous tumors are painless. I know for a fact, that testicular cancer is painless.
My question is, How old are you??? When my older brother, me, and a few of my friends hit puberty, we all got the lumps under the nips. They hurt like hell, and take years to go away.
Either way, it's scary none the less. Best of luck.