Started potty training 2.5 year DD 2 weeks ago. She's been brilliant at wees, rarely had an accident and asks to go. Trouble is poos, she's holding and holding till she's in agony over a couple of days. She shouts out I need a poo every 5 mins but won't just sit and go. When she eventually does she cries and cries even when it's out. Perplexed at what to do, she won't let me put a nappy on her now as she's in big girl knickers so I can't go backwards