I have suffered from that for years. Get the pain on in the upper quadrent of the knee just beside the kneecap. I discovered that one of those knee braces from the drug store - the sleeve type with a hole for the knee cap worked great. Tendonitis takes a long time to recover as they get so little blood flow. I found the brace seemed to help with that. I use one on any ride more than 40 miles. This condition kept me from being able to do a century for years - that is no longer an issue.
Good luck and I hope you find something that works for you.