Given you have other older kids, I think you've probably got good instincts about what's what's broadly 'normal'. I'm not saying there's anything going on cause I don't know. It could just be a phase or a behavioural response that she will grow out of but the fact that you're 'walking on eggshells' with her is a good indicator that her behaviour is pretty extreme. I think she needs an assessment. If for no other reason than you get another opinion and some peace of mind and maybe some new strategies to try. If there is something more than a 'phase' going on, early intervention now (or soon) will really make a difference in the long run.