Why does my son wake up crying as if he saw a monster?

Its happened when he had just turned one, then again last night and hes one and a half now. He will cry at night at like three something and its actually more like screaming than crying. Like crazy screaming and tries punching everything even us when we try to console him. I try to carry him to comfort him and even offer everything that usually calms him down, but he just shakes his head and screams even more. I just heard of night terrors but what causes them? I mean like what does he think is going on and what can I do if it happens again? I'm not the kind to believe in ghosts but when you see him do this, I think of anything. Forgot to mention that when I tried leaving him on the floor, he would crawl like a foot or two front, then back, still screaming and kept banging his head either hythe wall or on the floor so I picked him up instead and struggled with him instead. Hard part is I'm pregnant 38 wks and its hard to deal with him this way too