My cat's personality has done a 180 overnight

My 4 year-old spay female, Cleo, is normally front and center but very finicky about affection.  When she wants it, she'll let you know.  Other than that she watches everything from the same spot which allows her to see the living room and dining room.  If I yell, "treat", she's in the kitchen in a nano-second.  In the a.m. she's usually near our bedroom door (kept closed at night - no pets in the BR) and happy to see us along with our other 4 year-old spay female, Sophie, who has the personality of a doorstop but loves to lay in your lap.

This a.m., Cleo was no where to be found.  I finally tracker her down under the couch and she ran.  I can't find her now so she's behind or under something.  There is nothing amiss in the house and no vomitus, urine or feces anywhere.  The cat box is kept meticulously clean.

What's up with my little darling?

[April2]

It sounds like something spooked her. It's hard to say. How well do your two cats get along? Do they ever get into fights?

[Dumpbuster]

She was starting get back to normal last night and was 100% by 10:00 p.m.  Upset stomach maybe?  She's fine now, though.  Thanks for responding.  And, yes, the two do get along fine 99% of the time.