T.C.
It could be a whole slew of reasons.
He could be teething, could be going through
a growth spurt.
Make sure he's warm enough and for a night or two, you
may just have to let him cry through it.
My 2 1/2 year old goes through this just about every 4-6 months.
It's not easy, but if you give in to him everytime he starts crying, he's going to know that if he cries, you'll pick him up. It's one of the hardest things, I think, to let your baby cry through it. I never let my son go more than 20 min though!
That was my limit. If after 20 min he was still screaming/crying as much as before, I'd pick him up or go in & just rub his back till he fell asleep.
Good luck!