It could be a number of things:-
If it looks RED and swollen then you may have an infection (bursitis) on the joint (caused by your last injury). Other problems can occur from your old injury to do with the bone trying to repair itself, or arthritis.
It could also be anything also do do with other body systems such as heart or kidney problems but considering you have an old injury to the ankle, I would suspect this to be the most likely cause.
In any case, should your problem continue/get worse, become red/hot/etc, then please see your doctor. WIth best wishes, Matron X