How is your DS when feeding from the lumpy breast. Does he seem happy or frustrated? Does he fuss? Can you see him (muscle by his ear moving) swallowing? Do you do single sided or double sided feeding? Do you alternate which breast you feed from or is it possible you could be feeding from one side more often?
I do find one breast gets fuller than the other even now, this is very common. I know lots of ladies have one side with a faster let down than the other.
Have you mentioned this to your midwife and let her check your lumpy breast out? What about if you hand express does the milk come out ok?