Swamped

You're not seeing your own face. Actually, you can't quite make out what you're seeing. It looks a bit distorted and... reptilian?

You suddenly catch on to what's happening. You put the map in your pocket, reach onto your helmet, and pull down Mortimer the lizard. The angle you had the mirror at must have reflected him more than you.

Yeah, now that he's not there you only see the top of your helmet, with your eyes barely poking out under it. And it's hard to make out details of the helmet, at that. Maybe the mirror's slightly curved? Doesn't explain what you were seeing with the map, though.

"I guess you were in here after all," you mutter to the lizard.

"Here!" Mortimer declares excitedly. "Guess!"

So it wasn't Shume pulling a trick - he must have seen the map and grabbed it because he could. Which means it probably doesn't have anything to do with the mirror after all.

"Guess! Guess!" Mortimer continues, interrupting your train of thought. Does he want something from you? Does he want you to guess something?

"I'm not sure what you're trying to say."

"Here! Guess!"

The most obvious meaning you can think of is that there's something he finds interesting here and he wants you to guess what it is. Might as well play along - if that's not what he means, maybe giving him more words will help him clarify.