chronic/spreading pain

Hi, i'm new to all this, but i've been to a few doctors now and they still havn't given me any help at all so i thought i'd give this a go!

my left patella tendon started hurting around 9 months ago when i was jogging, the doctor advised a few weeks rest, and then i'd be fine (obviously not the case!)

my left knee joint then started to ache all over, and i was told i had mild tendonitis which would be cured with physio exercises and ultrasound- again i had no luck here.

over the last few months the pain seems to have extended to cover my left and right legs. the patella tendons underneath both knees are painful, both knee joints are extremely sore, and i have also been having a kind of burning pain (like the type when you get lactic acid during exercise) in my quadriceps and gluteal muscles.

I've had x-rays and an MRI scan which have shown nothing wrong, but it's now a chronic pain that seems to be spreading over my legs, and i'm worried that its just going to keep spreading.

Anybody have any idea what it could be? many thanks in advance

oh, also, im a 21 year old male who before this was very sporty and in good health, so it shouldn't be an old age related condition!