No, it really is possible, and sometimes easy to walk with a torn ACL. People tend to assume because it is such a serious injury with a lengthy recovery that it must be totally debilitating, and it isn't. It's counterintuitive, but, if you break your tib and fib, you're not going to walk. Your leg is flopping around. However, with that injury, you could be back skiing in as little as 6 wks, most likely 8.
Tearing your ACL, is the opposite. It can either be excruciating, or not, it can either swell like a balloon immediately after, sometimes very little, it can produce a loud pop, sometimes it doesn't, and for the most part, after the initial few minutes of pain, most people can walk away from it. As for swelling and ability to walk, that can vary based on whether you destroy your meniscus, and whether you sustain a nasty femural head bruise, or a tibial plateau bruise.
All this is to say that you should 100% go to the ortho doc and get it looked at. Everyone is hollering MRI, but the truth is, most doctors can diagnose ACL tears with a simple physical pushing and pulling on your lower leg/knee to something like a 90% certainty.