I would stop all sport for now until you have a correct diagnosis. No knee swells with 2 pints of fluid without an injury!! Sounds like the injury could have been extensive actually. Likely suspects are a cruciate rupture of some type or a patella dislocation which relocated under the gas. If the later were true then you might have in fact done both.

The recent episode suggests a cruciate issue as well, perhaps the anterior one. You should be able to get an idea if this is the case from the sports physio and this will be confirmed with an Wikipedia reference-linkMRI (as you cannot see the ligaments on a plain x-ray).

Sorry for the bad news but a brace should not be used until you have a diagnosis