Running A/C with extension cord into 20 amp outlet.

In addition to other good advice given, I'd also recommend turning off the breaker and pulling the outlet out of the box to make sure it isn't "stab" back wired rather than wired with the screw clamps where there's significantly more surface contact. You might be surprised to see how little actual contact there is in a "stab" wired connection. Scary when high loads are involved...