Brief Answer:
It can be due to heart, kidney or liver causes

Detailed Answer:
Hi! Welcome to health care magic!

Based on the history you have shared, since you didn't mention any pain specifically alongwith swelling so causes relating to increased uric acid or other joint diseases becomes less likely.

If swelling is painless, and if you press it above your ankle and a pit forms, for a while, it can be due to variation in osmotic or other pressures inside the blood which cause the fluid of blood to be pushed out of the vessels and get accumulated in dependant parts where it can gravitate easily with movement or walking to the ankles and feet.

By Pressure difference we mean heart pushes the fluid less and dumps it backward a little more, and this can be ruled out by an Echo cardiogram to measure the ejection fraction of the heart.

By osmotic pressure differences we mean any protien leak in urine due to kidney causes or decreased production by liver. For that simple investigations would be plasma albumin levels, urinalysis and liver function tests. Meanwhile keep good content of protien and egg whites in your diet. And keep an eye on any chronic history of loose stools.

If the swelling is pressed and pit isn't formed and remain swollen, you need to he assessed in detail by your GP to rule out causing leading to decreased drainage of this fluid by lymphatic system and blood vessels and thyroid levels may also be asked. Meanwhile raising it above the level of body while lying can help.

Purpose is to get you aqare of all possibilites and doesnt aim to create undue anxiety. And rest assured as most of these conditions can be cured or controlled with appropriate medications once you determine the exact cause but once such signs appear these need to be worked out.

Hoping you a speedy recovery. If you have any more questions, feel free to ask. kindly rate me generously and write a review if possible as a source of encouragement. Regards.

---