Help! Baby vomiting at night

Twice in the past week my 5 1/2 month old has puked when he woke up at night. This has occured after eating cereal about 1.5 hrs before bed (but we started cereal and purees at 4 months). The first time he slept 9 hrs straight and then when I went to go get him from his crib because he was crying he was covered in puke. It happened again tonight after he slept 5 hrs straight. On a typical night when he doesn't eat cereal he is up every 2.5 hrs. He has been exclusively breastfed besides the introduction of cereals/purees. What is going on? Is there something I can try/tweak and retain the long sleep hours without having my boy throwing up?