Hi
Welcome to the MedHelp forum!
A single ankle can swell in gout, rheumatoid arthritis, deep vein thrombosis, trauma or ankle strain. It can also be due to localized infection of the skin above and not be related to the joint. Since you have probably over exercised, it is possible you have muscle strain and something called compartment syndrome.
You probably need to rest the limb, elevation of the limb and some anti-inflammatory drug. However please do not start any treatment without consulting a doctor first. It is difficult to comment beyond this at this stage. You will need to follow this up with an orthopedic specialist. Please let me know if there is any thing else and do keep me posted. Take care!
I would go to the doctor and try to stay off your feet. You could have either sprung it, fractured it, pulled a muscle, etc..
I would recommend you have this ankle assessed by your physician.
I think you just overworked the leg.
You can always try elevating the leg, OTC medications for inflammation, ice and rest.