Please help.. I don't know what to do..

So the past couple days my son who is 2 months almost 3 (November 5th) has been really fussy all day crying and screaming. He is fighting his naps. I'm not sure if he is teething or maybe constipated? He drools a lot but also the day before yesterday he had a really runny diaper like his poop was soaked into the diaper and it was coming out the sides he didn't poop at all yesterday. So I'm not sure what I should give him? Or what's wrong. Anyone have any suggestions? He cry's pretty much all day for the past couple days and he has never been like that... I hate that I don't know what's wrong..