Wasn't there myself but a mate of mine went last year and said the same sort of thing - that heavy feeling in the chest when you get to the 5th floor, like the air changes. He's not someone who spooks easily either, works in a hospital so he's used to that kind of environment.
I've done a few dark tourism spots around the UK and sometimes you just get places that feel different and theres no good rational explanation for it. Waverly Hills has such a density of suffering attached to it, thousands of deaths over decades, so if any location was going to have residual energy that you could physically feel it would be somewhere like that.
What floor were you on when it hit you? And did anyone else in your group feel it or just you? That's always the bit I find most interesting - whether its shared or individual.