Step 4: Read and complete the decision guide to learn more about your symptoms.
From your answers (ankle pain for at least three weeks that is neither mild nor severe, with morning stiffness and swelling, but without other joint pain), a number of conditions come to mind.
A sprain could cause ankle pain like yours, although prominent morning stiffness would be a bit unusual.
Osteoarthritis could do this, especially if you are over the age of 50 or have had previous injuries to that ankle. Ankle pain in this condition is usually worse with walking.
Ankle arthritis may accompany conditions causing arthritis in other parts of the body (such as rheumatoid arthritis); past psoriasis, diarrhea or a sexually transmitted disease may be of particular importance to you (see arthritis associated with inflammatory bowel disease, psoriatic arthritis or Reiter's syndrome). Joint infection (including Lyme disease) is a relatively rare cause of ankle pain such as yours.
There are other possibilities (if you can believe it!) and early treatment is important for many of these, so it is important that you see your doctor soon for evaluation.
If your ankle pain is already improving, you may just need to give it a bit more time; over-the-counter anti-inflammatory medicines (such as acetaminophen or ibuprofen) are reasonable to try unless you are already taking them or your doctor has told you to avoid these. If symptoms persist, see your doctor for evaluation.