Pain in lower back and spine on bending.

I feel Pain in the left lower back and lower spine region and difficulty in straightening my back when I stand after sitting for sometime. The pain is also there when I bend forward. While bending forward I can bend more easily while bending towards right side, but I can't when I try to bend in slight left direction. After prolonged sitting, when I stand up then it takes a few seconds to get my back straightened and I feel the pain during this time as well. Pain increases when I have been more physically active throughout the day. The pain is like a tinge in the lower back and lower spine. It is more towards left lower back and I have difficulty while bending forward slightly tilted towards left direction, so I am worried. I have tried calcium tablets, vitamin d tablets, and some exercises as suggested by a doctor, but the problem is still there. It has been like this for more than a year. On days when I am always outside physically, the pain is much more while bending.