They may feel "strange" for a few hours or a few days, but if you give them attention and let them settle in, they are fine.
Its evident that they don't remember a long time, cause if you take the kitten back to the mother after a few months away - they will act like strangers to each other.
If your kitten is younger then 10-12 weeks old, its better to have 2 kittens from the same litter or another kitten/cat for a playmate. Otherwise you can have some personality problems and social problems.