What is this golf ball size hard lump near my mons just below incision site?

What you describe is consistent with a postoperative infection. You should report this to your plastic surgeon immediately.

Hello!  Thank you for your question. Your description warrants an examination by your surgeon. The signs and symptoms of redness, tenderness, and increasing size are worrisome and could be consistent with an infection/abscess or infected hematoma/seroma.  Hopefully this is nothing more than scar tissue or a suture, but these signs are concerning and should be evaluated. Best wishes.