18 month old is suddenly screaming and crying usually in middle of sleep or sometimes (very rare) right before bedtime. This has been going on for a week now and it lasts about 90 minutes to 2 hours. He cries, screams, throws himself around and nothing can calm him down. We cuddle him, soft talk, sing to him, talk to him yet nothing seems to calm him down at all.
Does anyone know what this is???