you could have a localized case of atopic dermatitis (eczema). it's more common than people think. i only get it on my legs, really badly on my left leg in particular. this area on your body just dries out more than others.
try this: take a shower, and while your skin is still damp, apply a good amount of lotion on your ankle, or a thin application of olive or almond oil works well, too. allow to dry a bit and put on socks dead last or not at all, if you don't have to. do this every day for 2-3 weeks and see if it clears up on its own.
don't scratch it, don't shower in very hot water, and make sure that if you're out in grass or anything, wear shoes and socks (this could be your trigger). if the bumps don't go away with this treatment, see a dermatologist.