R.J.
It's hard to sleep when you're in pain... either a head bump or teething could do it.
One thing I learned to do after 1 too many "head thwack!" moments (teething, etc.), was that if my son broke from his sleeping/eating schedule all of a sudden was to give him a dose of tylenol. 9 times out of 10, he went instantly back to "normal", and then within a day or two it became very apparent as to what was causing him pain (tooth would start to erupt, or a cold would announce itself, etc.). The few times he didn't, I knew to scratch pain off the list.
Teeth were especially hard with my son because he didn't get cranky with them at all... he was SUPER happy... and Always. Awake. Oy. But a single dose of tylenol let him sleep happily until it wore off.