My 3 year old daughter developed a strange light brown spot on her heel overnight. what could it be?

Chemical stain?: A light brown spot that appears overnight (not a tiny new mole) with no other changes of the skin surface, is most likely a chemical stain. Juices of certain fruits and plants can stain the skin, especially if exposed to sunlight. Another reason for spots is ink (markers, art supplies, etc.). If all else is fine, then the spot should go away in 1-3 weeks. A primary care doctor can check the spot.

Variable answer: This could be a bug bite, a rash, a bruise from stepping on something. I would give it a week to see if it goes away, if not then see your podiatrist for treatment.