Hard Lumps In Breasts. Mammogram Normal. History Of Breast Cysts. Causes?

Hi i have a hard lump the size of the tip of my pinky in my cleavage area for the 2nd time in as many weeks. i noticed one about two weeks ago it was larger than it is now but it went away/down within a week. And the pain was gone too. When i went to the Dr. i was told to that if it came back to come back in. Now i have another smaller one in the same spot with a little pain. which the first one started out small and then grew .... I recently had a mamagram and it came back good. what could the lump be... i ve never experienced anything of this kind in my breast before. I ve had a cyst before on my left breast but you could tell it was a cyst.