Unfortunately it's hard to say what that lump could be.  It could be a cyst, hormonal changes, a tumor of some type or even possibly a by-product of breast-feeding.

I found my lump near the 12 o'clock position of my right breast just weeks after a D&C (my 4th miscarriage and I had been in my 2nd trimester) I noticed the lump because I was pointing out to my husband that my breast were returning to normal when all of the sudden I felt a lump that felt like a marble.  I was hoping that it was a by-product of the pregnancy so I saw my ob/gyn.  She sent me out for a mammo and ultrasound.  The mammo ended up looking exactly like the pictures in the pamplet that they use to show a cancerous lump.  The ultrasound confirmed that it was indeed solid.  I was told I needed a biopsy or lumpectomy.  I had already scheduled a hysterectomy so I piggy-backed the surgeries and had the lump removed.  It came back positive but did not have clear margins.  I was told that due to the aggressiveness of my cancer a mastectomy was recommended ASAP, which I did the following week.

My words of adive for you would be to definitely have that lump checked out.  Until you can get in to see the dr I would take a black sharpie marker and circle where the lump was found - that way it makes it easier for you to track and to show your dr the exact location.  Your dr will probably want to follow up with a mammo and ultrasound.  Keep in mind that you have very good chances of the lump not being cancerous - but that it is always better to be safe than sorry.

Good luck to you!  I hope all goes well!

Angi