Hi! This is caused due to prolong standing in a heated surface. This is a kind of first degree burn, but no need to worry.
Next Steps
If you are feeling pain simply place your foot in a ice cold water for 2-3 minutes or you can use ice cubes and rub them in circular motion over your blister area. And for now avoid wearing slippers/ shoes and avoid prolong standing. Your symptoms may subside within 2 - 3 days, if not please physically visit a physician.