What you're feeling could be a cyst, or you might possibly have fibrocystic breast tissue (do you feel general lumpiness all over?)  Those might explain what you're feeling.  How long have you had this lump?  Lumps that do not come and go with your period or that last over the course of two or more weeks should generally be checked out by your doctor.  It's possible that this lump was detected on your mammogram, but did not appear to be anything threatening.  But if you're concerned I would suggest calling your doctor's office and request to speak with him/her.  He would be the one to tell you your mammo results, as well as request any additonal testing.  So rather than be worried, just make a call and find out.

I'm not 100% positive, but I do recall my radiation oncologist telling me something like my cancer will not be able to return to my chest now that I've been through radiation - but I can't say for sure because he has a VERY thick accent and I hardly understand anything he says - besides I've had a complete mastectomy so I have no breast tissue for it to come back to, and the recurrence or secondary occurrence I had was within my chest wall muscle - not something you can feel.

But I'm sure your doctor will help you any way he can.

Good luck!

Angi