Eating Regression?

So my son (21 weeks old) seems to have recently decided to go on some type of hunger strike...all kidding aside, he's barely eating 3 oz every 4 hours. He was eating 4 oz every 2.5-3 hours, however now just screams and cries whenever we try to give him a bottle. I am a little concerned because he's already on the smaller side since he was born early- only weighed 11lbs 9oz at his 4 month appt. I think some of it may be do to teething (he's been chewing on his fingers, being more fussy, drooling constantly) but as a FTM, I don't know much about this stuff. Is anyone else going through this or heard of this before? Thanks!!