What Causes Pain And Redness On The Bottom Of The Foot?

My mom has been seen by a doctor at a hospital with severe pain on the bottom of one foot. I saw that it was red before and still red but now with some light brown around and still very painful. The doctors at the fracture clinic wanted to see if there was any bones broken, and thankful enough no bones broken. They told her we don't know why you're getting severe pain. Can you help? She's also I forgot to mention Diabetic (up to 4 needles a day)????