Hi
Welcome to the MedHelp forum!
A single ankle can swell in gout, rheumatoid arthritis, deep vein thrombosis (ruled out in your case), trauma, fracture, ligament tear, tendonitis, bursitis or ankle strain. It can also be due to localized infection of the skin above and not be related to the joint. It can also swell if you are putting too much strain on it while sitting.
Since I cannot examine you and know other related conditions you may be having, nor is a detailed history possible on net, I have listed the various possibilities that should be looked into. Please consult your PCP for primary examination followed by proper referral.
Take care!