Just want to clarify if the breast lump was present before you started breast feeding or it occurred while you are breast feeding.
The lump you are palpating could be a galactocele which is common during or after lactation. In this condition, the ducts are blocked and the milk accumulates, forming a cystic structure. Try massaging the breast or use a breast pump and see if the lump disappears. You may also want to consult your doctor for evaluation.