Dd2 did this at the same age. It was night terrors, when they scream/cry/open their eyes but don't wake properly. It happens when they are transitioning from be sleep cycle to another. Most kids grow out of it.

Dd2 was worse if she was over tired or over heated. If its night terrors there is little you can do and waking them is very difficult. Gentle cuddles or having a big temperature change (like going outside) were the only things that worked for us

Hope it doesn't last too long