Might have just stepped weird and strained a muscle. If the foot is fully functional without severe pain then I would doubt you tore anything. Maybe a sprain/strain. Ice/Heat that mother and stretch it out every couple hours. See how it feels later.
Get a paper cup, fill it with water, freeze it and cut out the bottom so it looks like the end of a pencil w/ an eraser on the end then massage it with that. Usually works well for isolating a spot so you don't have to ice the whole foot.
Is there a large amount of swelling or any bruising?