I've been only walking at a medium speed on a treadmill, but after only 30 min I've noticed a feeling in my ankle that, if I were to continue, would become an actual injury that would prevent me from walking at all.

I wear running shoes and I'm not going above what I currently can take... what am I possibly doing wrong?

Maybe have your gait checked out in a clinic or running store - see a doctor if it doesn't go away.

Treadmills are notoriously hard on our bodies, so the easiest answer may just be to go for a walk outside instead!

Try to place ice on it

Or if you can get a robb, you can use it as well

This is a good one too. You may just try to see a Doctor