While sitting straighten my back get sharp pain in calf and back of thigh and sometimes in butt on left leg?

You're too tight. This is simply a lack of flexibility in your hamstring and calf muscles. You need to stretch both muscle groups in both legs 3-5 times daily, 1 repetition each side, with stretch held 30-90 seconds. Do this regimen for a month and see the benefits....Then continue it for the rest of your life.

Related Questions

Hiking uphill I get pain in my left inner thigh. Downhill is fine. After the hike, lifting my left leg up, I feel sharp pain in my inner thigh. Help!

Sounds muscular. Your description sounds like a strain of a muscle that travels from your pelvic bone to your knee, and whose action is involved with raising your leg. It may be a muscle strain of one of the left leg's quadriceps muscle. Read more...
Hip Pain. Concern would be hip etiology. When you go up hills you go into increased flexion at the femoral acetabular joint (hip joint). Many things can cause this including cartilage issues or osteoarthritis, labral changes, or femoral acetabular impingement. A clinical exam and simple x-ray may provide quite a bit of information. Read more...