Does sound a little like an obstruction, but you didn't say if she was in pain. Obstructions are extremely painful. You will have labor type pain that comes in waves much like labor does. You will feel the pain around the belly button area.
I would ask for a referral to a Pediatric GI. Explain that you have a strong family history of autoimmune diseases and would like a colonoscopy ran. That would be the starting point. At least with the prep she would get cleaned out really good. I don't know if they have Miralax for children, you might want to try that to help clear her out some too. But I think the first place to start is with a colonoscopy. And normal bloodtests are not a sign that everything is okay. I have been sicker than a dog and my blood tests come back normal. Good luck and keep us posted.