Top of your foot
If there are red spots on the top of your feet, they can be a sign of Systemic Lupus Erithematosus (SLE) which the body's immune system mistakenly attacks healthy tissues. The red spots are the condition that immune cells attack the capillaries, causing hemorrhage under the skin, called vasculitis.
How to differentiate from frostbite or rash is:
Press the red spots with clear plastic spoon and if the redness is gone, it is not serious but if it is still red with pressure, it can be hemorrhage under the skin so it is recommended to go see a dermatologist.