I'm sure it's a phase. My DS did this also at around that age (a pretty good sleeper at night usually before this 'phase'). The night you described last night was exaclty like the one we had with DS the night before DD was born (so no sleep and then labour all night the following night!). He was 17mths. By 2, he was back to sleeping just fine, but I recall going from newborns room to his room and back to newborns for a few months there....... Hopefully your phase will be short lived.

ETA : I never worked out why either, but it must be age related! Probably molars as someone else suggested.