3 year old holding in poop!

My 3 yr. old has been potty trained(pee only) for a while now but she has never gone poop on the potty. She will always go in a pull up without a problem. Well about 2 weeks ago she had the flu pretty bad and she didn't poop for almost 8 days. When she finally went is was painful. Now, she is holding in her poop and saying she is scared to go. She won't even go in a pull up. I am giving her a capful of miralax a day and have changed her diet to reduce cheese, milk, etc... She has always had issues with pooping. She would only go every couple of days. It is hard because her favorite foods are cheese, yogurt, bananas,etc...which don't add to her problem. Well today she has gone 6x so far. I guess the miralax had finally kicked in. She screams and shakes each time and says she is scared. I am so frustrated and of course think it will never get any better. Anyone who has had experience with this some input would be greatly appreciated. I hate seeing her in pain and it just started out of the blue. the miralax has made itso she can't hold it in, which is good but I don't want her to be scared to go.
thanks