Step 4: Read and complete the decision guide to learn more about your symptoms.
From your answers (mild ankle pain without fever or swelling, lasting at least three weeks, no other painful joints, with morning stiffness), a number of conditions come to mind.
A sprain could cause these symptoms.
Ankle arthritis may accompany conditions causing arthritis in other parts of the body (such as rheumatoid arthritis) though there is usually swelling and other joints are usually painful.
Osteoarthritis could do this, especially if you are over the age of 50 or have had previous injuries to that ankle. Another possibility is sarcoidosis, which may cause inflammation near the joint (including tendonitis) rather than actual arthritis.
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.