Hard to explain… loose/ wobbling knees, pain below left knee, aching, etc???????

Im am 23 years of age and have been a postman for 5 years and doing a round for the last 3 years.
I cant exactlty remember when the problems started with my knees but now it has got to a point of desperation!
I had always carried my delivery bag on my right shoulder, which as you can imagine would put more pressure on one side of my body than the other.

The problems started with my left knee it always felt as if it was out of line so was constanly grinding and clicking etc i also had a problem with my left shoulder.

I eased this discomfort with some special insoles which i got from the hospital which gave me the support i needed.

The problems i have now is that when i am standing still (sorting for long periods) both knees are extremely un-comfortable they feel as if they are extremely heavy, like there being weighted down, i suppose the muscles are more relaxed when standing still than when walking.??
When i am walking a long it feels as though my knees are wobbling about but still heavy, is this making sense its hard to explain?????? Maybe there is some sort of increase in fluid around my knees??

Both knees seem to have a lot of movement when im walking, im trying to strenghen them up with swimming etc.
My left knee has an added pain on top of all this.
Normally when sitting/laying down if i try to tense my leg up i get this strong shooting pain below and around the knee, it kind of feels like its bone on bone pressing together, could this be that ‘PATELLAR TENDONITIS’ I keep reading about?
Ive been told by my doctor just to take Aspirin or Ibruprofen for a week or so and see how i get on…

Another point i have to make is that when i have different material trousers on, ie; jeans irritate my knee, light wheight joggers dont irritate them as bad.
Any ideas advice would be greatfuly appreciated.

Rob.