Both of my knees suffer from pseudo locking, and have for about three years. tendonitis?

Locking of Knees: Usually locking , unable to straighten the knees is due to meniscus or cartilage tear. You are a little young to have this going on in both knees. However, if you played hard, or have arthritis in the family it is possible to have torn meniscus. You may need to see an Orthopedic doctor if they continue to do this. Sometimes arthroscopic surgery may cure the problem.