Baby vomiting yellow?

i have a ten day old son who is vomiting yellow the vomit is projectile. he does this once every other day. i took him to the hospital where they did an x-ray to check for more serious conditions this came back clear so my baby was discharged and was never diagnosed of having anything wrong. since he has been discharged he has been sick again i am just wondering if anyone has had any similar experiences with their babies. the thing thats unusual is the fact that it is only happening like once in two days