It's probably nothing to be worried about - but, get it checked by a doctor just in case.
Where you are in your cycle can explain lumps, too... your breasts can be more lumpy around your periods.
I know it's easier said than done, but try not to worry too much until you know that it is cancer. I found a lump myself and worried myself sick about it... it turned out to be a cyst that comes back there and in other places - and I need a course of antibiotics to get rid of them sooner than they go away naturally.
I have one of these cysts now.