I agree with the arch support. I had this and didn't take care of it quickly enough and it brought me down to 20 mpw last indoor and ruined my season. Thinking back I think i may have gotten it from starting to do all of my strides barefoot whenever I had never done that before.
The usual RICE treatment is necessary. What helped me more than orthotics (which I got fitted for) is an arch support tape job. If you don't want to completely stop running, then you will have to get arch support or you will keep injuring it every time you go out. If you have access to trainers ask them for an arch tape job, they'll know what you mean.
If not, you can go buy your own tape at Academy or somewhere and do it yourself.
http://www.youtube.com/watch?v=6wfmOmJRSio&feature=related
It takes a bit of practice but I got pretty good at taping myself this way after about 3 tries. Of course I had trainers to teach me how.
Like the poster above said, you need to gradually get away from the arch support when you can.