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.

The area where the drain comes out frequently becomes red and irritated after a few days and sometimes can heal much slower than the rest of the incision. That is why most surgeons bring the drain out through a separate site although unfortunately that creates a separate scar. If you do bring out the drain through the incision then its probably better to avoid the very ends because excess tissue or what we call "dog ears" can gather there. I believe that its what you're seeing, a combination of a small dog ear that was irritated by the drain. It will most likely"settle" with time and is nothing to be seriously concerned about. At worst if you have a persistent excess of skin (dog ear) present after 6 months they can easily be removed with a little local anesthesia.