Eastern Hognose

Hognose play dead as a defense mechanism, and that includes holding their mouth open and sticking their tongue out. They also flatten themselves out almost completely to look larger to a predator, also another defense mechanism. I promise this is an Eastern Hognose. If you just zoom in on the picture you will very clearly see the upturned snout, it can't be anything besides a Hognose.

I'm really glad you left this snake alone since it is a harmless Hognose. You can clearly see the very pointed snout, and the tail curling is a very common thing this species does. They are highly variable in pattern, this just happens to be a very dark one with faint bands, but it is definitely not a Cottonmouth.