With any change like the one you describe, the best thing to do is see your primary care physician
right away. He or she will want to discuss your recent activities in more detail--although you are right, it does not sound like there is a specific trauma that is to explain for these changes--as well as review your medical history. Your age and prior medical issues may make certain causes of knee pain more or less likely. The most important thing, however, is for a physician to examine your knee. Depending upon what he or she finds, your primary care physician may want to refer your for some imaging studies and possibly evaluation with an orthopedic specialist. Your symptoms sound like you could possible have a problem with your patella (kneecap) or the meniscus within the knee joint itself. If the patella is partially subluxed (not seated properly) or if you have a partially torn meniscus, your knee may feel unstable and you may have difficulty with bending. An exam will be very helpful here because your physician will be able to do specific maneuvers to test the strength of the knee in multiple directions to help pinpoint where the likely deficit is at.