I agree, it sounds developmental if you can't find a medical/pain cause. My pediatrician says that night crying/waking can be a sign of ear infection in young kids.
Dr. Brazelton's bookTouchpoints talks about "developmental waking". It sometimes happens when your child is doing a new developmental skill...i.e. starting to walk, talk. It usually doesn't last very long, few nights while child is focused on skill? It sounds like a pain scream, but it is just a weird sleep/brain thing...and sometimes they aren't even awake. I tried to comfort my kids in their room, or not go to them if they weren't too frantic...see if they could self-sooth back to sleep, afraid that they would get used to me coming in at night and get in the habit of waking on purpose.
From what I understand, true night terrors are usually from being overtired. Like if baby is refusing naps, or going to bed too late. When one of my kids was 4 she did that when transitioning out of naps, so we had to put her to bed by 7 pm.