Bruised ankle

[Deleted User]

Can anyone help ? I have what I think is an odd problem. On longer runs (approx 8 miles+) I am getting a bruise developing on my right ankle. It about the size of a 10p on the back of the bone (achillies side). It is uncomfortable when running and very tender when I stop. It is visibly bruised but after a couple of hours the main tenderness goes away but can linger slightly for a day or so in a minor way. The bruising stays for a couple of days. I have changed my shoes recently, but they are identical to my last pair. I don't remember any trauma to my ankle - even a minor one. It doesn't happen on shorter runs of about 6 miles or less. Does anyone have any ideas as to what could be the cause or how to stop it happening ?