What Can Cause Swollen And Painful Quads While Playing Basketball?

I'm 41yrs. old and play basketball 2-3 times per week. I feel pretty good most of the time with the expected soreness. Lately I can feel my quads ache/throb with every step and have noticed weakness in them. It doesn't feel like normal soreness, just little strength and pain in quads. What is it and what should I do?