Why does my knee hurt so much after walking/running?

I started cross country about 2 weeks ago and my knees were fine then, but now they hurt like crazy! It has been like this since monday of last week. When I walk a little bit, my knees are fine, but then when I walk for longer distances or when I start running, or going up and down the stairs my knees really start to hurt! I've noticed that during a cross country meet the first couple of steps were awful, but the ones after that, it felt like my knees sort of went numb. I have a meet on monday, so it would be nice if I could figure out whats wrong before then. Any ideas?

you are not used to it duh.
keep doing it and if you don't start to get used to it and you still hurt, talk to your doctor and consider reevaluating your diet.