sitting up

Katie
My daughter is almost 8 months old and she literally hates sitting up!! She is just now starting to sit up on her own. Every time I try and sit her on her butt ahe stiffens  up and wants no part of it. She loves to be on her stomach and rolling around. She's just starting to crawl too. I just don't get why she hasn't mastered sitting up yet . Should I be concerned?