Tennis Elbow, or Lateral Epicondylitis, is soreness or pain from inflammation or injury to the OUTSIDE elbow. Golfer's Elbow, or Medial Epicondylitis impacts the INSIDE elbow.
It's usually around this time of year when it happens, the change of seasons when players are transitioning from outdoor to indoor tennis, or har-tru to hard court, that's when your elbow starts to hurt.