I had the same thing with my (now) 2 year old. Everything was
fine and then he started 'waking' in the middle of the night screaming. My husband and I realized he wasn't really awake. Then we found out about night terrors. Part of the difference is that the child is still asleep with night terrors.
We solved the problem with a couple of things. First of all, we eliminated spicy foods from his diet. (The night terrors started when he was a little older than your child). Then we made sure he wasn't too warm at night because night terrors in Auyervedic Medicine (Deepak Chopra) are associated with too much heat.
Then we started playing classical music very softly while he sleeps.
He hasn't had any problems since we implemented these things and we were relieved because he had suffered from them for a few months...he usually had them a couple of times a week.
Anyway, good luck and best wishes.