I have this thing inside my foot and I don't know what it is?

On the part of my foot that is right under my toes, kind of like a heel, but the top part, there is this ball like this that appeared on the inside of my foot about 2 months ago. It hurts so bad when I touch it and when I walk on it. It's completely on the inside of my foot and it kinda looks like a bubble. It is extremely painfull and I dont know what to do. PLEASE help me. Thanks so much! =]