While driving i can feel heat near my feet?

92 caprice 88,000 miles i've been driving and realized i can feel heat in my feet and also my passengers say they can feel the heat. its summer time and i dont feel the heat when the ac is on only when its off and i've been driving for awhile. My temp. Gauge isn't work and remains at 0 so i cant tell if its over heating. The car doesn't cut off and runs strong at night (75 degrees temp) but a little sluggish during the day 85+ degrees. What could be the problem?