What could be wrong with my knee?

I took my daughter sled riding a couple weekends ago at the local country club. As I went down the hill I was sitting on my legs with my knees touching the sled, well there was a little ramp that some kids had made out of snow well it rained then it snowed and needless to say it froze a little bit so the ground was hard, well I hit that bump, not meaning to because I never saw it until after I had went down the hill, anyways my knees hit the ice and it was a hard a a rock, well they kinda both popped but after a few minutes I got up and walked away, well it's now maybe a week and a half later maybe a little longer, I worried I might have messed it up, but I have to pay when I go to the Doctor and it's not cheap so I thought I would just take it easy and see how it felt later but my left knee now locks up when I walk ( it didn't do that before) and it locks up to the point where I have to stop and hope there is something to grab on to and I get sharp pains in it and it's still a little sore, What could be the problem, I know I should go to the Doctor but that means X-rays and all that, so I just kinda want to get some advice first~