What Causes Severe Pain In Feet?

I have severe pain in both my feet. My doctor says it is planters facitis, but i honestly dont think it is. I tryed physical therapy and cortazone shots and nothing worked. I can't stand on my feel for more than 3-4 minutes without extreme pain, and it is freezing cold out and the only shoes i can wear are flip flops! I would love to know what is going on.