Curiosity

I am always curious why there are some nights Elijah will go to sleep in less than a second and there are other nights when he will cry for hours on end.  Most nights he goes to be right away.  To night is not one of those nights not so great nights.  It's odd, he will be laying up there, then all of a sudden bust out crying for no real reason.  I think he might be cutting teeth, again...or he is getting a cold.  Either way I don't like my options.  Fussy baby.