Yup - totally normal! I remember DD was an absolute dream child for about two months (7-8 months), then it all turned nasty again, LOL!
She has been asserting her independence ever since
I see it as a positive thing, really. They are growing in awareness of their world, and while it's frustrating (because they're incapable of caring at this stage about how YOU might feel about something) and can be exhausting (they don't necessarily remember or understand WHY they are told 'no') it is fascinating to watch them develop. Enjoy this amazing time!