My 19 months old son wakes up in the middle of the night screaming in pain. He arches his body n throws himself around in pain til he passes gas. Why?

Constipation? That sounds miserable for both of you! Constipation is exceedingly common in this age group, and it may be difficult to determine how severe it is. It can cause these symptoms--but there are other causes. Best to have your Pediatrician examine the child in-person, and see if testing or treatment are needed. If your child drinks more than 16 oz milk per 24-hours, that's a good place to start.
Odd pattern. The usual reasons for night screaming is not pain but a night terror or nightmare.Night terrors usually occur in the first hours of sleep and do not wake immediately if you touch them,it stops after a few minutes.They usually have poor sleep habits and a sleep deficit. Nightmares are common in early morning hours and the kids awake immediately if touched. I can't put the gas as meaning anything.