Need Help. Mysterious "Butt" problems

I'm mostly a lurker here, but I really need help with this one. I have 7 and 4 year old daughters. They have butt problems. In all seriousness, the comments of "my butt hurts" started to crop up a year or so ago anytime after they pooped. Then it seemed like a stalling tactic to get out of bed. "Oh my butt hurts" comes up only at bedtime etc. I became concerned though when they would occasionally wake up at night crying about it.I've been playing detective in the mysterious case of the hurting bottoms. Everything looks healthy and as it should for both girls, no redness, rashes etc. We've gone though proper hygiene and wiping. I've asked if anything happened that hurt them etc. I'm thinking to rule out the unthinkable. They have been in my sole care since the end of school and before that they went to different schools so they were not exposed to the same caregivers. As a last ditch effort I treated them both for pinworms about a month ago. I thought surely that had to be it. After the treatment, the complaints did seem to stop for the most part but now it's happening again. My 4 y/o just woke up crying and flinching, pissed off because "her butt hurt". Niether of them seem to be able to give me anymore info then that.It's so frustrating, it angers me that something is going on with them and I can't fix it. I feel like this, of all things, is going to be my parenting fail. Help us please! via /r/Parenting http://ift.tt/29i2bex