My dog started yelping suddenly...?

I'd have him checked to be sure but if you were all out of the room and going away both times then its possible he may be feeling bummed out and looking for attention, my own dogs have learned to act like theyre practically choking for attention once they know barking won't work and they feel the other is having more attention than them, and somebodys learnt to pretend it is having a fit for attention as unfortunately its sister died of a fit when it was young and it learnt through watching her