What is this nerve pain behind my knee?

For about a month now,I have been having shooting nerve pain behind my knee (posterior to my knee cap) and sometime in my thigh and calf on my left leg.I have read some reports about people having sciatic nerve problems but i am NOT experience any tingling or numbness.It really only bothers me if i am standing or walking for a long period.I haven't hurt it running or anything it just started hurting one day and hasn't stopped since.Anyone else ever experience this and what could be done?