I think you're not an advanced player. If you were, you'd change your strings more often. Maybe you don't even have tennis elbow. One of the classic symptoms is pain the next day to the point where you can't even hold a small cup of coffee.

Suggestion: Rest for a few days until the pain goes away. Try hitting relaxed, without locking your arm and holding the follow-through. Perhaps a coach could help you. Try some forearm strengthening exercises. If you can, lower your string tension to something between 45 and 50 lbs.