What would cause a 13 month old girl to throw up all of a sudden for no apperant reason

My daughter is 13 months old and for some reason once, every other month, she starts to throw up and won't stop unless she gets prescribed medicine to help her stop. She doesn't have colic, she has reflux but takes previcid for it everyday in the morning. We have taken her to the emergency three times in the past 6 months and all they do is give her something that helps her stop throwing up and watch her drink a bottle of milk to make sure she doesn't throw up again and sends us away with no reasonable explaination. They have done ultra sounds and x-rays to see if there is any blockage but everything comes up negative. I would greatly appreciate any ideas from anyone, thanks!