Are you saying that the knee actually catches, won't completely extend briefly and then pops into full extension?
No. It extends just fine. It doesn't pop. It feels like it needs to. For ex: I just stood up and tried to lock my knee and it feels like it needs to pop but it wont and it hurts. Its hard to explain. :)
No not stiff. It feels like its inside my knee ( kind of the front/middle)
Hmmm....It feels like its more towards kneecap.
OK. As you can imagine it's a bit difficult here since I can't examine you and the symptoms (not your fault) are a bit on the vague side, but you may either have a problem with bursitis or something called chondromalacia patellae which is a problem with the cartilage behind the kneecap. I'd suggest an MRI of the knee and perhaps some physical therapy and maybe a cortisone injection.
Thx. What about exercising? Any suggestions on cardio exercises I can do that will not further aggravate my knee?
Do you know what could have caused this sudden onset?
If it's bursitis, which I think is most likely, that can occur without any obvious cause. For now, cycling, either with a bike or on a machine, swimming and exercises in water up to your chest, abdominal crunches are all fine.
What about yoga?
Thank you for your time and information.