Don't feel bad about not knowing the sex! I'm a breeder and my one rex had ONE white baby in a litter. I thought it was a boy for weeks - looked like it to me and was gonna keep him. When you only have one kitten and nothing to compare it to - it can be a little hard. If you know genetics and know the parents color, you can determine sex a lot easier...its when the color can be either sex is the problem
Anyway, kitten was doing fine, until I took him to the vet's for 1st shots at 7 weeks old. Vet came in and said to me "are you sure its a male?" I said yes, I breed cats, I know it is.........vet: Well your little boy really is a little girl!
Boy was I embarrased - first time I couldn't tell the sex, but I swear the kitten had balls in the back!
One other thing that is a little bit of a problem in only kittens. They never learn proper sibling lessons. They can have a problem in adjusting to other cats. Even tho I had other cats in the house, she was a little weird with them - as if she didn't know how to play properly.
She also turned out to be deaf and was placed in an only cat home where they had a deaf dog. Kitten got along fine