Pin behind the knee
is unlikely to be due to varicose veins
and is more likely to be due to knee related issues. Without a physical examination and probably a few tests, it would be difficult to pint out the cause in your case. Possibilities include infections/ inflammations, nerve impingement/ pressure, tendon/ ligamental tears, meniscal injuries, fractures, patellar dislocation, bursitis, tendonitis etc.