Hi Kezia,
My DS does something similar. He sometimes starts crying and/or shouting suddenly in his sleep, and he does it since he was a newborn! What we usually do is go to him, put a hand on his chest and the other one on his head and that's enough to calm him down.
Someone once told me that with the amount of new things a baby learns and sees every day, it shouldn't surprise us if they actually had a bad dream once in a while. Now... this is not scientifically proven, just an opinion from another mother.
Hope this helps!