I've Had A Small Blackhead On My Breast For Years.

I've had a small blackhead on my breast for years. Every month or so I will squeeze and the blackhead come out but latey I've noticed when I squeeze brown stuff comes out and it has a bad smell. I have also felt a large lump in my breast in the same side. Had a mammogram in October and they said it was only a cyst but it feels bigger. What to do?