I keep going back and forth on whether my 4 month old baby has acid reflux or not.

I keep going back and forth about whether my 4 month old baby has reflux or not.  He often strugles to feed, feeding best when sleepy, arches his back and turns his head into the nursing pillow (I didn't realize what he was doing, but now I wonder if he is trying to posture himself to feel better).  Is generally happy as long as I am entertaining him, but can have random violent crying jags where nothing makes him feel better, sometimes refusing to nurse as well.  He does not like to be in the car seat and screams as if someone is stabbing him the whole time we are in the car.  He tolerates the bouncy seat and swing momentarily, but often scrunches himself up in a sit up like position.  He did the same thing when swaddled and is now sleeping on his stomach.  He can sleep up to 8 hours straight at night, but only cat naps during the day.  He is gaining weight well and doesn't actually spit up although I often see old curdled looking milk in his mouth.  He is exclusively breast fed.  I have tried Zantac, which gives him diarrhea and currently am trying to give him Prevacid when needed as this seems to hurt his stomach too.  Not sure what to do, does it sound like reflux.  Should I take him to a specialist?

Additionally, he chews on his hands so much he often gags himself.  Could be teething as his sister got her first tooth at 4 months, but then I also read that reflux babies do this.  He also often seems like it is painful for him to have a bowel movement, crying before he goes, but when he does go they are runny and yellow like breastmilk bowel movements.