How long have you had this and where exactly on the ankle is is
the outside or the inside of the ankle
any discoloration to the area? Is it painful?
Did you bang it on anything? Any bugbites that you can remember?
Ok. This is likely a fluid collection within or about a tendon sheath - however to definitvely make out what it is, either an ultrasound or an MRI of the ankle would be needed ultrasound would show if there is any vascularity to the swelling (which I doubt from what you've said) and an MRI would show from which space (a tendon sheath versus from within a joint capsule or a fluid collection not associated with any specific structure) Where it is arising from can only be definitively determined by that. And treatment depends on where it is arising from.
Your primary care doctor can order ultrasound and MRI (their choice obviously which to choose first - ultrasound would be more cost-effective)
And if they'd see fit they could send you to an orthopedist.
If this is generalized calf swelling or redness or that begins to develop then I'd recommend going to the ER, however from what you said that does not seem the case.
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