They Begin To Accuse You Of Cheating
If all of a sudden they accuse you of cheating and you can’t wrap your mind around it, guess what? It might be them who is cheating on you. It often happens that when someone cheats they knowingly accuse the other person of cheating meaning they “project” their character onto you.
So if they cheat it means that you are also capable of cheating (in their mind). It is just how human psychology works. Be wary if they start accusing you of anything that you aren’t doing. It will be bad for you if you don’t want to lose them and start feeling guilty for no reason in order to please them.
It will only get worse and worse until they break up the relationship. After a week or two, they will be with another person and you will wonder how that came about.