Is this normal from an 8 year old girl or am I doing something wrong?

More details would be useful, but remember there's a huge amount of hormonal changes occurring.

Without anymore details as ben asked for, I can only say that my own 8 year old is very moody. At 8 a lot of girls bodies are starting to prepare for puberty

I do large classes of 8 to 10 year olds, and moody is a understatement at times.

Has this been a gradual change or rather sudden?

Somewhat gradual but a lot more sudden the past few weeks. She's been more mean towards her 6 year old sister and has more attitude and meanness towards me as well. Has even told me she hates me. She cries easily and is hard to satisfy. She can't make up her mind for some of the smallest things like what to eat. If we go out to eat she'll b the only one to complain of where we're going but if u ask her what she wants she'll just say "I don't know". She's very nice and respectful to others very good a smart in school. Her teachers don't believe that she can b this way towards me. They say she's very kind, helpful, and respectful and organized. U should see get room it's a disaster and I have to help and bribe her to clean it.