If a person has bad circulation then even a walk up a hill can cause swelling, equally in both ankles. If this same person though, were to do this walk up a hill for seven days in a row, then the ankle would no longer swell up surprisingly. As the body is getting used to exercise, the circulation becomes much better.
If this has not been caused from a walk up a hill however, then a thorough investigation is needed.
Aside from fluid having gathered in the ankle, another possible reason could be that there is inflammation around, or in, the joint.
A doctor can take your blood sample as well as your urine sample to find out the exact problem, but here are some of the most common reasons why an ankle could swell:
A sprained or broken ankle is definitely going to swell, and even if the sprain wasn’t severe, swelling will occur to some degree, as will a stress fracture.
Arthritis in the ankle is not as common as any other degenerative change in your other joints. Ankle arthritis is very painful and is found in people who have rheumatoid arthritis or someone who has previously injured their ankle joint.
A collection of what is known as uric acid crystals that are in the actual fluid of the ankle is known as Gout. A person with gout has an abnormal amount of this uric acid and it causes swelling and inflammation inside the joints.
Blood that is not flowing properly can block normal circulation and cause the swelling. This is known as a Vascular Obstruction and forms a blood clot which is also called a DVT. This usually happens in only one limb, rather than both, and causes swelling in the soft tissues in the ankle and the foot.
Another circulatory problem is a peripheral edema, but this happens in both the legs. It can be due to aging veins and causes chronic swelling from the pressure put on one's legs or when pregnant, obese or if you have varicose veins.
Lastly, an infection, although rare in the ankle, can cause swelling and is healed with medication and sometimes surgery.