This morning he texted me "Have a good day!", I didn't respond, and then he texted again "How was your class last night?" I responded with short "fine, justa long night".
I think it's also the use of the word "just" here. It's as if you're justifying YOUR distance from him, when HE'S the one cheating, and you are only protecting yourself emotionally!
That "just" is used as a softener, MB, as a way to "rescue" him from feeling the consequences of your delayed response.