This is probably not related to sciatica at all. It could simply be tendonitis
, or a mild irritation of the posterior capsule of the knee
. It could aso be a Baker's cyst, which is a collection of fluid from the knee, which herniates and collects in a "sac" behind the knee. Worst scenario is a deep vein thrombosis, DVT or blood clot, ehich can be very serious. Therefore, if it persists, you should see a doctor.