Hi. Lumps at the back of the knee may suggest a growing cyst in the skin that becomes more prominent with changes in the positioning of the legs, a series of dilated veins in the skin that enlarge after exercise but then decrease in size after exercise has been completed, or a small defect in the tough fibrous covering around the muscles of the leg that allow muscle to herniate through it during exercise. For a more definite diagnosis, see your physician. Thanks for asking.
The lump can be any number of things! I suggest seeing an Orthopedist
Thanks for comments. Your seems like what it likely is. I will head to the doctor on Monday to get it checked out.