I second the plantar fasciitis guess, which is something I suffer from. It feels like I've been beaten on the bottom of my heels with a hammer sometimes.
I deal with mine by using high quality insoles with a higher arch support (because I'm sort of flat footed), stretching my legs and feet thoroughly before and after playing. Icing the feet after playing to reduce swelling, and taking ibuprofin after playing to reduce swelling and pain.
Mine is not completely well because I refuse to quit playing long enough for it to heal (sure my doctor would advise I stop playing for a while), but the stretching before and after and use of good arch supports helps greatly.
With mine the pain was worst first thing in the morning. Getting out of bed and getting downstairs first thing in the morning was excruciating. stretching would loosen my feet and make an immediate improvement.
Hope that helps.