If the pain was definitely there before she stopped pooing, then it doesn't sound like it was that causing the pain. As thumbwitch said, if it hurt to poo because of the side/backpain that could have caused her to withold, so it could be a red herring. Also, if she was seriously constipated or impacted it would have taken a lot more than a big drink of water and a satsuma to shift it.
You're not useless at parenting, you are just worried about her, its perfectly understandable, you are absolutely right, constant pain in a child isn't right and shouldn't be ignored.
I would second trying an osteopath if it does turn out not to be intestine or appendix related. One of my ds's, my Mum and myself have all really been helped by a really good osteopath. You can go on the General Osteopathic Council's website here and search for a registered osteopath in your area.