Sounds serious. Go see a doctor immediately. If your foot is not broken (negative x-rays), then you could still have soft tissue damage. Either way, seems to be getting worse, so go see your doc.
Welcome to the MedHelp forum!
A single ankle can swell in gout, rheumatoid arthritis, deep vein thrombosis, 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. X-rays will be negative for all causes except fracture. MRI of the joint may be required. It is difficult to comment beyond this at this stage. You will need to follow this up with an orthopedic specialist ASAP or go the ER.
Hope this helps. Take care!
Could it be a muscle tear?
If the swelling is what they used to term 'fluid on the leg', my mother had an old wives tale to treat it. I broke my kneedcap (kneecap got pushed round to the outside of my leg then pushed back into place, but apparently a couple of pieces had broken off the kneecap). My knee immediately swelled up and I couldn't walk on it so my mother got washing soda in a pillow case and wrapped it around the knee overnight, in the morning the washing soda was a hard lump and my knee had this powdery stuff on the skin, but the knee was markedly smaller, after three days of this the doctors were able to operate,
I thought it was my mother being weird - but it worked and several friends have now used the system on their swollen joints.