Played tennis and in outfield. What caused my elbow pain and how do I fix it?

It depends. If the pain is medial and goes into your ring and little finger, it's probably cubital tunnel syndrome, which is a pinched nerve on the inside of the elbow. If its the outside of the elbow, it may be tennis elbow syndrome. Both should be treated with rest and anti-inflammatory medication, but if it keeps hurting see a doctor.