Why does my knee hurt?

I have a job reading utility meters and two weeks ago we had a lot of snowfall and and ice. while walking in it for about 20 hours in a two day period my knee started to hurt severley. I was slipping a bunch while walking and the pain started the second day as more of a really sore feeling i made it hard to walk like my left leg was real stiff, since it was a friday i thought resting over the weekend would make it better. It did by monday but then started to hurt again now after a weekend it feels better but as soon as i get to working it comes back. It hurt directly in the middle of my knee cap and down the outside of it along my leg, seems worse after sitting for a while and when it hurts fells almost impossible to move my leg due to the pain, but the weird thing is the pain goes away almost like it never happened but comes back and it hurts more walking down hills or down steps than going up. but both can be extremely difficult until my leg seems to loosen up. any ideas