Ankle advice please !

[Ant50]

Up until now I have been running three times a week, covering distances between 5-8km, without any issues. However, I’ve called it a day at 4km on each of this weeks runs as I have had some soreness/stiffness in both my ankles. Not easy to explain but the ‘ache’ is across the top of my foot at the ankle joint, it just feels very stiff. I find if I stop and do some extra stretches it helps for half a km or so, then it’s back. By the time I hit the 4K distance I also notice a very slight ache in the back of my calf’s muscles (low down toward the ankle). Nothing else has changed in terms of running shoes etc and I haven’t twisted my ankles or anything like that, likewise I don’t think I am heel striking etc.

Just wondering if anyone can offer any advice on ankle strengthening exercises or maybe exercises to improve ankle flexibility, as maybe I need to spend longer stretching/flexing my ankles before running. Thanks.