At first you think you’re imagining it. Or you explain it away as the person having a bad day or an off week or something going on in her life that’s affecting her. Because it’s someone you’re cool with, you want to give him the benefit of the doubt, want to consider every possible explanation for his weird behaviour before you confront him. But as time passes, or incidents occur to which the person’s reaction is wildly different from what you’ve come to expect from them, you eventually realize that something has definitely changed. You don’t know what it is, but you know it has.
I like to compare this phenomenon to walking around a room you’re pretty familiar with. You don’t know it like the back of your hand, but you’ve been in it enough times to know what’s good. You know the layout, have a pretty good idea where the doors and electrical outlets are, you can find your way around. And then suddenly the lights go off and you’re in that room in complete darkness, fumbling around trying to find everything that was so accessible just a moment ago.