baby grunting?

My baby is 17 weeks and just turned 4 months officially today. He's been extra fussy all day and has been grunting on and off all day. 
It started early in the AM today. He's not hungry and he isn't constipated. I'm not sure what could be wrong? The only thing I can come up with is teething? He was fine yesterday. Not really fussy and normal. Then around 11pm last night the fussiness started...