I have a big lump under my left armpit after delivering a baby?

Breast tissue/glnd : In that location the most common lump would be an enlarged lymph gland. One rare possibility would be stray breast tissue. If you draw a semicircle from the armpit thru the nl breast location along the mid belly to the groin on the same side, you have the line where accessory nipples(usually duds) may be found. I had one mom that had an accessory breast tissue in both pits arise after delivery.