could also be plantar fasciitis, or a tearing of the tendon/ligament extending from the balls of your feet to your heel.
I had this a few months ago and I had to resist playing for a few months to let it heal. A few trips to a specialist later and I was almost good as new.
If it is PF, its most likely from 1) not enough arch support in your shoes or 2) too much strain placed on the foot for an extended period of time.
schedule an appointment with your doctor and check out www.webmd.com
for further help
(oh, and don't get cortisone shots. They hurt like hell)