Why is there pain when i stretch a muscle sometimes?

Not unusual: Mild discomfort with stretching is not unusual when muscles are a bit tight. Avoid rapid, jerky movements with stretching- stretch smoothly and slowly, and don't "bounce". If you have discomfort with stretching, don't push further; holding briefly is ok. Stretching routinely should improve flexibility and decrease pain. If pain persists, a strain or tear could be present, and may need evaluation.

Muscle pain: You have to be careful when stretching a muscle. Do not stretch a muscle when your body is cold. The muscle will not be elastic and stretching a stiff muscle can cause pain. Try to do a light warmup and or at the end of your training before you stretch. This will allow the muscle to be more flexible and less susceptible to pain. Also do slow sustained stretches and not quick sudden stretches.