Pain in left arm just above and below elbow. Less pain elbow bent. No help with rest. Seems like muscles. What is cause, how to relieve the pain?

Ice it. It can be related to repetitive movement. Ice it for 15 to 20 minutes for the next 2-3 days, if no improvement seek medical attention. If you have no history of allergy to anti inflammatories, you can try ibuprofen, take with food. If the pain worsens, you develop numbness, weakness or have associated chest pain, go to the emergency department for further evaluation.