If your doctor "has no idea" and lets it go at that it may be time to find a new doctor.
It could be very serious. This, from WebMD:
Heart, liver, or kidney disease. Sometimes swelling can indicate a problem such as heart, liver, or kidney disease. Ankles that swell in the evening could be a sign of retaining salt and water because of right-sided heart failure. Kidney disease can also cause foot and ankle swelling. When kidneys are not functioning properly, fluid can build up in the body. Liver disease can affect the liver's production of a protein called albumin, which keeps the blood from leaking out of the blood vessels into the surrounding tissues. Inadequate albumin production can lead to fluid leakage. Gravity causes fluid to accumulate more in the feet and ankles, but fluid can also accumulate in the abdomen and chest. If your swelling is accompanied by other symptoms, including fatigue, loss of appetite, and weight gain, see your doctor right away. If you feel short of breath or have chest pain, pressure, or tightness, call 911.
Read the entire article here:
What Causes Swollen Ankles and Feet? - WebMD