My 15 month old keeps waking up at night....why?

My 15 month old was sleeping through the night perfectly from age 6 months up until a couple months ago. Some nights are fine but others......he eats three meals a day of baby food and oatmeal. It happened last night 2 hours after I put him down. He ate a big bowl of oatmeal but refused a bottle before bed. He also refused it when he woke up, he seemed very upset by the bottle which is odd because normally he loves it. He was up for about an hour with me laughing hysterically at nothing. The laughing isn't normal, I don't know what that was about but he wakes up like this 2-3 times a week and I don't know why. I can't let him cry back to sleep because he screams and screams until he throws up. What do I do????