I've had some experiences with knee injurys so heres just my say:
Just because you heard a pop doesn't mean you tore your acl. I had the exact same incident happen to me (I was a lineman, got blindsighted by two guys, one going at my knee one way, one going another) and I felt a pop. It was extremely painful for the first 1 or 2. Then when the trainer looked me over, the pain was gone, and all that was there was a loosness in the knee. Turned out I had pretty severely torn my MCL which is on the inside side of your knee. heres a pic (LCL is the ligament on the outside, MCL is the torn ligament in this picture, and the ACL is a ligament that crosses like an "X" on the inside of your knee):
Depending on where you were hit on your knee, you may have suffered from several different injuries. It could be a torn MCL, LCL, or (better hope it's not this) ACL. I suggest you get it checked out ASAP, and stay off of it, lest you aggravate it more.
And just a heads up, if you've torn ANY ligaments, the minimum you are gonna be off of it or in a brace, is six weeks. If it's bad enough you may even require surgery. But surgeries on torn MCL's and LCL's aren't that common, so hope for one of those. ACL's require extensive surgeries most of the time, but it depends on the tear.
You may also have something wrong with the miniscus, but thats rare.
short version:
GO TO THE DOCTOR