PT and withholding poop...

Scuba_Girl

DD asked to wear big girl underwear and go on the potty as of a couple of weeks ago, so we started PT.  She's doing pretty well with peeing, but has only pooped once on the potty.  We were so excited about the poop, we took her to Build A Bear and everything, and she seemed happy and excited...but since then (last Wednesday), she doesn't want to poop on the potty, only in her pull-up or underwear. 

So I told her it was ok, she could go pee in the potty and poop in the pull-up for now, but to tell me when she was ready to go poop on the potty.  So now she has been holding it in sometimes, and today she became constipated.  After some prunes, prune/apple juice, and Benefiber, it came out, but it was painful for her.

Did anyone else's DC withhold bowel movements?  If so, how did you handle it, what worked for him/her, etc.  How long before they stopped doing this?  My DD is 2 years, 8 months.  We're hoping to have her fully potty trained before September so that she can go to preschool.  TIA!