I agree with Alice, a 2nd opinion might be a good idea since your 1st Dr. was not an ortho. specialist. Not trying to tell you what to do or anything but I had a similar experience.
The first Dr. that looked at my knee was a general Dr. at a ski resort hospital clinic. He said I didn't tear anything and had a bad sprain. I had NO pain, and a little swelling and instability like you so I was pretty pumped and believed him. After 10 days of resting it there was little improvement so I went to an ortho. specialist. All I did was mention instability and he ordered an MRI. Unfortunately in my case it turned out to be completely torn ACL and severe meniscus damage. I was told I could wait to have surgery but it would leave my knee super vulnerable if I did any activities beyond walking. So if it is more than a sprain, riding too soon could make it way worse.
Good Luck
(Message edited by parkgirl on June 10, 2009)