I not sure of your age and history, such as have you had children etc.. And I am NOT a Dr. so take what I say with a grain of salt.. I had a simular thing happen and it turned out to be an infected milk duct. This was found with a mammogram and breast ultrasound. I did nurse my children, but the problem occured years after my last child was born. Surgery was suggested but I chose to just "live with it". Still am. It does get smaller and cause less irritation a lot of the time, but seems to worsen Pre menstrual. I also have Fibrocystic brest disease which causes benign kumps to just form out of the blue throughout the month. Some are painful and some are not. I have been told that the "perfection" in the shape of the lump is a good sign. If it is round and smooth then it is most likely a benign cyst filled with fluid. It is the grainey feeling unusually shaped ones that are more worrisome.
Now with all that personal info that you didn't ask for said, I really do urge you to see your Gyno. Anytime you find a lump in the breast you should have it checked, just to be on the safe side. A mammogram at any age can serve as an easy, affordable, lifesaving tool. Soo , please go for an appt. to put your mind at ease. Good luck.
oh my word! that seems pretty big and ANY odd lump or bump definitly needs to be checked out and dont wait. is it a hard lump or squishy? do you have any discharge coming out of your breast? there is also a breast forum. im sure you will get great advise here but you may want to post there as well. let us know ok?
Get it checked out it takes 5 min. out of your day. I never let it go even it im sure its nothin im not REALLY SURE.