I scared

A: "Mommy! I scared! I scared!"

Mommy: "A, It's bedtime. What are you scared of?

A" I scared of monsters. I scared of monsters."

Mommy: "Excuse me?"

A: "I scared of monsters. There! Monster there!"

Mommy stands there perplexed as A starts crying hysterically and cowering in the corner of her bed.

Mommy: "Monsters? Really? "

A: "Monsters! There! Turn light on!"

Mommy: "OK, OK. See, no monsters!"

After 15 minutes or so, I finally got her settled down, but I remain perplexed. I'm unclear how she learned she should be afraid of monsters. We haven't watched and shows about monsters except Sesame, and while Elmo may be scary to me, he's not exactly the stuff children's nightmares are made of. We haven't read any books featuring scary monsters. This seems to have come out of the blue.

C has never had a scared of the dark or monsters phase, so I am a little unclear how to proceed here. Any thoughts from been there done that parents?