My right ankle has had mild stiffness and ache for 3 months. any idea of the cause?

No: It's ok to ignore for a few weeks, but anything going on for months should be checked out, could be inflammed ligament, could be bone tumor. It should be evaluated by doctor.

I agree: If using rice and nsaids does not relieve your symptoms you should be evaluated. If there was a specific mechanism of injury it may also be a good idea to be seen by an orthopedist for an xray and evaluation.