That exact same thing happened to me a few years ago. It WAS just tendonitis. You'd think the high degree of pain would imply a terrible serious problem but it didn't at all. It's good to have your doc check it, but take heart in knowing it may not at all mean an awful thing. My doc, a major athlete herself, said, "So don't kneel on that place on your knee, then..."
"My predominant feeling is one of gratitude. I have loved and been loved;I have been given much and I have given something in return...Above all, I have been a sentient being, a thinking animal, on this beautiful planet, and that in itself has been an enormous privilege and an adventure." O. Sacks