I'm A 145 Lb, 5'8" Female, Aged 63. I Am

I'm a 145 lb, 5'8" female, aged 63. I am in excellent condition with low bad cholesterol and high good cholesterol. I have blood pressure of 120/60. I play tennis 20 hours a week....hard core tennis, not senior tennis. Lately, I have been have severe dizzy spells that last long enough to put me out of my tennis match or make me pull over my car. I have no cold/sinus issues so can't imagine that it is vertigo. Any good guesses?