Fussing and moaning then passes out..

Any ideas?

FTM here, baby boy sometimes fussy and is niggly and seems hungry so I try feed him but he pushes the bottle away (aswell as a pacifier) he continues to fuss and be niggly but then all of a sudden falls fast asleep...... I dont know why he does this? Am I missing a cue or something???

Help a girl out!!