Is My Boyfriend Seeing Another Girl Behind My Back?

Go with your instinct. If he's starting fights and not spending time with you most likely he's done moved on. So please don't waste your time. Go on with your life. If he doesn't come back around then he moved on and didn't care too much for you or the relationship. If he comes back here have a couple of choices you can ask him what was going on and work on the relationship or I would show him that that was messed up what he was doing to you and goes around comes around. Let him know that you are independent and that you went ahead and moved on. The way I see it if he wasn't there when you needed him the most then he's not going to be there for the long haul. Think about it.

I agree with sammy. You should go with your instinct and find out what is going on. After that then do what you think is right.

You are not a crash dummy he can't use you when he want to. If you feel that he is with someone else  you should just leave him alone you can do better. Even if he is not seeing anyone else he don't need to treat you like that.

Ask him what his problem is and then move on.

You can always tell when there is someone else, men give themselves away. If he is being distant to you, then start being distant to him. Act like you don't care, don't text him or call him, let him do those things to you. If he doesn't notice a change in you and turns things around then you know he is all in with whoever it is he is seeing. So at that point you just move on.