It's not the heat; it's the humidity.

Currently the relative humidity is 91%. I has not been below 75% all day.
 (There's a lot of sciencey stuff here, but they break down the basic premise here: "For example, if the air temperature is 75 °F and the relative humidity is zero percent, then the air temperature feels like 69°F. If the relative humidity is 100 percent at the same air temperature, then it feels like 80 °F. In other words, if the air is 75 °F and contains saturated water vapor, then the human body cools itself at the same rate as it would if it were 80 °F and dry")


And...

Everything feels damp.