By this time, I hope you had follow up with your surgeon. "Hard lumps are uncommon after an unevently lat flap. Women commonly feel their chest cartilages which feel hard. The muscles, skin, and fat are easily examined by an experienced plastic surgeon.
Thank you for your question.If you feel a hard lump, you should seek medical attention by your oncologist. You need to be examined and tests need to be performed (imaging, labs) to make sure you do not have any recurrence of disease. Best of luck to you...