5 month old projectile vomiting after every feeding ?

My daughter is almost 5 months and has been projectile vomiting after every feeding. She was born preemie and I was told it would stop around 1 to 2 months. I've noticed as she gets older its gets worse. She'll also spit up/ throw up several times after feedings and this can last up to 2 hours after she eats and she'll vomit bad at least one to three times. We tried one acid reflux medication and it slightly worked but gave her a stomach ache and just made it take longer to throw up. After that the doctor had us get a upper gi series done but couldn't find anything. We've tried switching to soy and that still didn't work. She'll sometimes act like its bothering her but other times she'll act fine. But as she gets older its becoming worse. No one has any clue or idea what could be going on, including the doctors.