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.
@ArishMell Yes, I am aware that men can and do get breast cancer, too - I tend to mention that more during breast cancer awareness month because it's just as important that he keeps an eye out for what is normal for him, too and most men that I have known don't even know what they should be looking for - never mind how often.
Similarly, there are also women who don't know that their breasts might be more tender and lumpy at that time of the month... so getting word out there and educating people is important - regardless of gender.