Pain behind my knees/walking.

Alright, I have been walking ever since I got my shots in late November. I have had a steroid injection to my hip area and also a Botox injection to the piriformis muscle and have been walking almost every day since but I have constant pain behind my knees. Now I am aware that some of these muscles haven't been used in a long time but here it is February and I am till having pain behind both knees and it doesn't stop. I use Voltaren gel (it helps) but that medication creates side effects with my system (I use the minimum) and Celebrex is worse. Aspirin does nothing, hot soaks don't help and I can't use cold wraps, I can't tolerate the cold. Any suggestions?
The pain is right behind my knees in the back of my knees in the muscle area it feels like. It feels like a stretching and my muscles don't like it. Muscle relaxers do nothing. I have three of them and none of them do any good. What more can I do?
I take a break from the walking every two days but the pain comes back after walking. What more can I do?

[TheMadMan]

I was only asking for some type of professional advise here, hope that's not too much to ask.