Tremor inside elbow, when bend my arm, causes please?

It is not feasible to provide a meaningful opinion without additional history, physical examination and may be some tests. It would be prudent to see a doctor, preferably a neurologist. Wish you good health!

Probably what you experience is fasciculation (muscle twitch), not tremor, and it is uncommon to people at times. However, if symptom persists and becomes diffused and/or some associated symptoms such as weakness occur, you should see a neurologist by then.