My Kitten is scared of everything.. Why?

ok so here's the details,

i have 2 kittens, litter sisters, nearly 8 months old, i have had them since a very young age, and am the first human contact they have had so they have ever been treated badly.

ok so Evie is the perfect kitten, she very playful, always sits on my lap, sleeps on the bed, loves being stroked and belly tickled and you can pick her up and hold her for ages without as much as a wiggle!!

Tilly on the other hand doesn't like being picked up, well you can pick her up to move her, but sooner or later she wants down, she sits to one side and watches when Evie is playing, Tilly has her moments when Evie is asleep etc but nothing much, she sleeps on the floor next to my bed when i turn the lights off but doesn't mind a stroke while they are still on before i go sleep, but will not stay on and when she sleeps downstairs when im sat down she will sleep in the corner hiding away, although she walks over to me when i come home after work, shes not scared in that sense and not scared of people, but going outside (just the back garden) shes scared of plastic bags, any real noise, and i only have to get the hoover out and she is upstairs hiding. i can understand why she may not like it.. not many cats would, but Evie is fine with the hoover, but my main issue is that after ive finished and put the hoover away she still cowers and is really scared for a good hour or 2 afterwards and wont eat.

they are just completely different cats, every time i so much as touch a plastic carrier or bin bag she runs off and is really on edge!! much more than normal. why does she act like this? she used to sit on my knee for hours, follow me around all the time and was really playful, maybe taking a slight back seat to Evie because she jumps around like a mad cat, but they were always playing and sleeping together when they were younger, sharing their food bowls with each other etc

neither have ever been treated badly, and have always been treated equally i just find it very strange why she is so frightened all the time

i have been living at my new home now for 2 months and this has only been happening for the last few weeks, can someone please tell me why?? she loves the new house they both played for hours a day when we got here and it was great, so i don't think shes stressed at the move or it would of shown earlier..

am i over reacting and she will grow out of it or is she just a scaredy cat??

thanks in advance!!