We were adopted by a kitten someone dumped in our neighborhood in August 2009. She was about 6 months old at the time. She was very friendly toward pretty much everyone, including the vet. She got spayed and we got her health issues handled.
About 6-9 months after she found us, she started getting funny about guests coming over. It started out as minor hissing and has since escalated into fierce hissing and screaming. She doesn't usually leave the situation though; I usually have to pick her up (hissing and screaming) and carry her upstairs to her room and close the door. (She has a room to herself upstairs with a cat tree; the dogs know not to go in there.)
At home, with just us, she's fine—sociable, friendly, curls up on our laps purring, etc. But if we try to go somewhere and I get my petsitter to take care of her, it's extremely stressful to her. Petsitter is very experienced, used to work as a vet tech and has several cats of her own.
The last time she was at the vet's she had a complete meltdown too; the vet was the same one who saw her at 6 months and had even written down in her chart what a sweet kitten she was.
As far as I know, no guests have ever mistreated her, intentionally or accidentally. I keep a pretty close eye on my pets when people are here, mainly because my hound dog can make a nuisance of himself (he never gets enough attention, you see).
So should I just preemptively put her in her room before guests come over? Should I talk with the vet about pharmaceutical options? I really hate trying to plan a trip knowing she's going to be traumatized every time our petsitter stops by to scoop her box and give her fresh water, etc.
Is her name Sybil, perchance? I'm so dating myself-
Maybe try clicker/treat training w/your sitter? Sceer'd kitties are so tough to treat-when I worked as a vet tech, I dreaded the bad kittehs much more than the nasty GSDs, etc...so hard to hold-they seemed to know just when your fingers began to cramp and then they'd tornado... ninja kitty