What could this be please help!!!!?

I have never had a foot injury! About a year ago My heel really hurt and i wore a brace to bed that was my mom's. That made my foot feel much better. Now my heel is hurting only occasionally. I was wondering what it might be! It only hurts if i do alot of walking and that is only sometimes too. I could run 2 miles and it won't hurt. I am in great physical shape. Also, I love to walk and run. I live for gymnastics, dance, and swimming and when my foot hurts it makes me want to push past it but i wnat to know what i should do!