You have to first get off the ankle and asses the situation.
Is it swelling? Is there bruising?
Go see your GP and get an x-ray done.
Unless you have recently sprained, twisted or taken a fall of some sort, and this ankle pain is persistent, it is probably because, like me, you walk funny.
By walk funny I mean you do not distribute the weight evenly when walking through the soles of your feet. Also, the muscles of your feet and the muscles supporting your ankles are weak.
If this is the case, please do not self prescribe - it'll probably make it worse unless you are an expert. But I would recommend low-impact exercises like pilates - with a proper instructor.
Hope this helps.