Waking through the night

For the first time in months, Cohen did not sleep well at night.  It wasn't that he would go to sleep and then wake up crying....I would rock hiim to sleep and put him in his crib and the minute that he touched his mattress.....he would cry.  We finally put him in bed with us and he slept through the night (from 2 am on).  Needless to say, I got no sleep.  

He is still in bed with Eddie right now.  It is crazy being a parent, I just wish I knew what changed last night to make him so upset when I put him in his crib.