My kid is holding her poop?

She takes a daily sup. of fiber but she still dose not want to go. We have to force her to sit on the toilet all day long but still nothing. It's been 5 days. She is squeezing her together as hard as she can. This is really frustrating and scary i don't know if it will hurt her. She does this every month or so then she'll be fine then refuse to go again. I've tried the tub and other meds. It's all in her head. I tried to tell her everyone in the world poops and sometimes it may hurt a little but nothing is working. I keep saying try or we are going to the hospital but nothing is working yet.