Is it allowed to change the game that different from the prev game i won?

It is not generally common or accepted to send a different game than the one for which a giveaway was created, though there are obvious counter-examples (e.g. giving away a "complete pack" instead of the stand-alone game because the GA creator didn't realize there was an option for the complete pack as well).

What game did you get instead of, I presume, PixelJunk Monsters?

Mara.

You can't change a giveaway after creation AFAIK. But if you weren't gifted the game you won that is an entirely different matter.