Waking up like this is scary

So it’s the 2nd time this has happened. I just got woken up by someone crying and screaming “Mommy!” This time it was a more faint, like it was far away. After a few screams, I jumped up saying “Hold on!” and ran into the other room where my son is passed out. So it wasn’t him. It was definitely a boy and it was a boy screaming.

I’m wondering what this means! Twice now! Am I afraid for my son? Is something going to happen? Yes, it freaks me out this much. I want to say it was a nightmare but since all I heard was that, is it technically a nightmare?

Any thoughts?