i had a similar problem, if you check out my thread on this. i first felt it doing a pushup. and then when i tried to play tennis i would notice it on a forehand only. and once when i goofed around with a frying pan serve.
i only had X rays done, no MRI, X rays found nothing. doctor thought it was a torn ligament, he suggested resting 3-6 months. i rested 4 months and i've been back on the courts twice with no pain in the hand at all.