Jodi,
Sorry about the fall. While the impact may have caused a slight amount of injury to your knee, I suspect that it was not the main cause of your knee injury. The reason is that your knees did not have very far to fall before impact, and they probably weren't carrying any significant amount of weight, at the moment of impact. IOW, your upper body impacted much harder than your knees, (and probably independently of your knees).
This is just a wild guess, but I'm guessing that you may have damaged the lateral collateral ligament, (LCL), when you tripped. If that is what happened, it would probably have been due to the oxygen hose applying a force that twisted your lower leg, or pulled it sideways, (inwards,toward the other leg), thus straining the LCL, since the LCL is on the outside of the knee, and it's job is to try to prevent the knee joint from flexing, (buckling), sideways, (outwardly).
I'm familiar with that particular type of injury, because my left knee is sensitive to that. A couple of years ago, I was foolishly using the outside edge of the sole of my boot, to scrape gravel back into a rut, (in too big a hurry to go get a shovel), and I accidentally overdid it, and flexed the knee joint enough to tear that ligament. The pain was severe if I tried to put any weight on that leg, and I had to use crutches for a while, but there wasn't much swelling. It took at least two weeks before I could walk without a severe limp. Since then, I have to be very careful that I don't apply any side loads on the outside of my foot, (or the inside of my knee), because that knee is very vulnerable to that type of injury, now, and I've had a couple of minor "re-injuries", since then. Here's what I'm referring to:
http://www.nlm.nih.gov/medlineplus/ency ... 001079.htm
I hope it improves quickly.
Love,
Tex