Projectile! HELP

We have a 7 month old preemie (3 months adjusted…) We of course saw the Gastro Dr. yesterday and we were having mild problems with vomiting periodically with feeds, we made some minor changes to his medications, he is currently on Prevacid 1/2 solutab 2 times a day and Erythromycin 3 times a day… he was doing well with the combination until a few days before we have to refill the erythromycin so we changed that to every 10 days instead of 14 days because we think it is losing its effectiveness….

so since seeing the gastro yesterday we have projectiled every feeding since with this horrible hacking sound…. I don’t know what to do! the hacking continues even after the projectile…. and this is all NEW!~