Fussy 5 1/2 month old!!!

My son will be 6 months old February 1st. The past 3 days, today being the 3rd day...he has been so off his usual self. He is super fussy...especially as night. He is still sleeping good. He sleep from about 8:30pm-9pm to 3am (when his daddy gets home) and then from like 4am until 7:30am. He’s been on this same schedule since about 2 months old, and he naps throughout the day. Anyway, the past couple days

Nothing we do seems to make him happy. He’ll play for like 15 minutes and get bored. He has been eating less not SIGNIFICANTLY less but definitely less. And finicky with his bottle too. He is constantly chewing on things, his hands, toys, blanket...bottle nipple...etc. and he is drooling. So I’m guessing maybe teeth? I feel them every morning and nothing sharp yet....I can’t see in there good enough to actually look good. They do feel swollen though. Anyone know what it might be? Did anyone else go through this?