They told her to "look for the big guy", and she legitimately did not know how that was supposed to help her until she found... the big guy.

Holy crud he's huge.

And he's definitely not human, as far as she can tell. Is he a monster? Then what if the power that brought him back to life made him...

She doesn't get to finish that thought because he's looking right at her.

"Uhhh.... uhhh??" It takes her a moment to respond. "N-no! Nothing like that! I j-j-just... you know... there was probably a lot of, uh, magic? Magic! Involved to, um... bring you back from the dead... and all, and magic can be kind of un-unpredictable?"

Smooth, Alphys.