pain in the arch

I'd taken a nearly month-long hiatus from running partially to concentrate on getting the bedroom renovations done, but also because I was having issues with the arch of my left foot. After a particularly trying week littered with frustration-venting runs I developed a fairly sharp pain in my left arch, closer to the heel.

Through the rest of the day, it got a little worse and now I'm sitting with one sock on, icing the arch of my foot.

Any idea what the deal with this is? Any suggestions for treatment/prevention? Maybe it's time to get fitted for a good pair of running shoes.