My money's on exactly what winkill and poiuy6 said..........plantar fasciitis.  Very common, and very painful. Check out this link from Mayo Clinics and the out-links from their site.  You'll gain a lot of info and ideas of what to try.  Good luck, I literally feel your pain. 

http://www.mayoclinic.org/diseases-conditions/plantar-fasciitis/basics/definition/con-20025664