Vomiting toddler when laying down

I'm assuming it's possible he just ate more than usual and his tummy was too full... Normally it's pretty hard to get my (almost) 2 year old to eat anything really, but today he ate great (which we were shocked by)... Although 4am I was woken to him vomiting in his sleep... I sat him up and gave him water and he acted fine... I had him up for a half hour and he wasn't even acting sick... so I laid him back down and as soon as he was on his back again he started vomiting. I feel so bad and I don't know what to do for him... he has no temp, and when he's upright he seems 100% fine. Any ideas??