T.H.
Yeah, Jennifer brings up a good point...does she think it's a game? I mean do you think she is genuinely terrified or not?
This is very odd, I'll give you that. Maybe you made a funny/scary face that you didn't mean to make and didn't even know you did make and it freaked her out. Who knows.
If she's verbal I would try asking her next time she does it and see what she says. I would also maybe try saying things like Mommy didn't mean to scare you and I'm here to love you and protect you or something and see if that helps. I'm guessing (hoping) she'll get over it quick, especially if you stay home with her.