Sounds like a simple overwork injury to me, but...
Gently rub the skin over your patella. If it feels like you have small grains sand in there, you may have prepatellar bursitus. My Dr. wanted to inject cortisone, but I protested (some studies showed cortisone could weaken tendons). He subsituted an anti-inflammatory and it cleared up and never returned.
Except for road rash, the only cycling injury I ever sustained!
Also...simply drink more water. This has alleviated knee pain for a number of my cycling and backpacking friends.
Sometimes the simple "solution" is the best.
Hope your knee feels better soon!