I have a lump near the semitendinosus by the back of the right knee. i body-build. no pain. slightly smaller on left leg. any suggestions?

Consider : An ultrasound. You may have a popliteal cyst (or cysts) and this is an accurate and relatively inexpensive way to look for one.

Cyst?: It sounds like it could be what is commonly called a baker's cyst or popliteal cyst. A baker's cyst is a buildup of joint fluid (synovial fluid) that forms a cyst behind the knee. If it is small and painless, simple observation is sufficient. You should have your pcp or an orthopedic doc examine you to confirm.