Strange lump/knot on scar where the drain was after full tummy tuck with muscle repair? (Photos)

This looks like a slight bunching of skin at the end of the incision.  Somewhat irritated.  Does not look infected or any cause for concern.Discuss these questions and concerns with your plastic surgeon.Best of luck!
Dr. SubbioBoard-Certified Plastic SurgeonNewtown Square/Philadelphia, PA

The  tummy tuck operation is optimally performed with minimal discomfort and maximum return to activity as promptly as possible. Drains significantly impede this recovery and can have postoperative sequela which can be challenging or even difficult to manage. Adapting surgical technique with modifications  such that drains are superfluous improves patients experience and results. Contour deformities where drains are removed can be revised once they wound is fully matured and healed. Check with your plastic surgeon for timing of this. 

I suspect this  is just inflammation from the drain being there for a while. Your surgeon would be able to rule out any other issues. This would improve over time, but a low potency steroid injection later on may speed up the process. 
BEST REGARDS, Prashant Soni M.D.