The poop may scare her. My daughter was having this experience. (Argh. It sucks on so many levels.) I was told by the doctor and many good friends to give her Miralax. It will soften the stool. One friend of mine had to give it to her son (once a day) for a month, while a friend of hers had to give it to her daughter (once a day) for six months. So far, I've been giving it to my daughter for about 2 weeks. We have no more crying and she actually got up last night to tell my husband she had to go potty (poop & pee). It was after 11pm when this happened.
By the way, I have been extremely cognizant of her diet. I've cut back on milk (and she only drinks Silk). I rarely give them candy and they drink more water than juice (and we are big juice drinkers). And as you know most 2yo's don't like vegetables, so I've had to get creative in the kitchen. A little bit at a time but it's helping. Also, my 4yo eats the same things and drinks the same things (but she's on Lactaid) and she doesn't have a problem using the bathroom (very regular).
So if your little angel is holding back it could be because of one bad experience. You can also try soothing her while she goes, or giving her a book, or putting on some of her favorite music. I always had to flush the toilet a few times and rub my little angel's back and thighs.
Good luck to you mommy.