Nine years ago, my husband and I adopted two kitties: Molly and Agnes. Agnes has never weighed more than 6 pounds (2.7 kilograms) and is what our vet calls a "snuffler." Because she started life as an alley cat, she has a virus in her lungs that makes her breathing sound labored. Agnes had a rough start, and when we opened the cat carrier for the first time to introduce her to her new home, she hissed at Molly, then spent two months hiding from us. For Agnes, the world was a scary place full of mean dogs, cruel people, and not enough food, but after living in our house she's come out of her shell.
A nervous kitty is often head-shy, meaning she will shy away if you try to pet her on the head. She'll also be prone to hiding, and you might notice her slinking -- walking with her legs bent so she's low to the ground. She might also twitch her ears or lower her tail with the very tip curved upward. Our nervous kitty grooms obsessively, and when she's agitated she will sometimes groom until she has bald spots on her back legs.
It's hard watching her feeling so scared! A little bit of hiding and nervousness is normal for any kitty in a new situation, but when nervousness persists for more than a few days it can be stressful for you and the cat. But you can help teach her that she's safe in your home and maybe even with company over time. It just takes some patience, a few training tricks, and a lot of love.