What Causes Leg Cramps Upon Getting Up From A Sitting Position?

This started a couple of days ago i got up from a sitting position and i was forced to sit back down when a horrible pain occured suddenly in my left leg.The front of my theigh and the back of my calf were hurting horribly, simultaniously. The pain comes everytime i stand and has now started to bother me when i lay down as well, making it difficult to sleep. The pain seems to only subsides when i sit in a particular position. Over the counter anti-inflamatory medicine helps but only for a short while and no matter what i do once i stand and begin to walk the pain comes rushing back. It is almost like a simultanious cramp in my thigh and calf but it is very painful. No reddness to the leg, no swelling, no temperature. Any ideas. 31 year old male with high blood pressure.