18 month old crying a lot at night ...

[HopeNope]

18 month old who was a good sleeper is now waking up many times at night and crying a lot. Appears to be in pain but I can't figure out what could be wrong.

He does have new teeth coming out but he's had molars out before and never been this unsettled. It also just appears to be at night.

What could it be?

Could it be the 18mth sleep regression?..

Sorry if this isn't at all relevant but just thought I'd post as our 18 m did this last month. Is there any chance he's cold? We realised the radiator needed bleeding and got the extra layers on, and it was all good.
It's awful though - sympathy!