Daughter not wanting to stay with Dad

I''m just wanting some advice please. I have been divorced 17 months and we have 4 children. Our older 3 kids are young adults but our youngest is 8. There has been a lovely little routine in place for her to see her dad during the week and she stays with him every other weekend where he would pick her up at 9.00 am Saturday morning and bring her home Sunday evening. Her last 2 occasions to stay with her dad she has wanted to come home at about 10.30pm as she is complaining of tummy ache. Neither of us want to see her upset and as he only lives 5 minutes away, he has brought her home. Now those of you that know my story also know that my ex lives with my best friend of 20 years ( and yes, she too is now an ex!) I find it strange that after a set routine of 6 months, she is now wanting to come home. We''ve talked and I''ve asked her tell me if anything is wrong but she says there''s nothing wrong and she wants to stay with Daddy but she just gets a tummy ache. Do you think it could actually be her instinct kicking and churning her little tummy over in telling her this "situation" is wrong but she isn''t old enough to interpret it???
Her older 3 siblings don''t really have anything to do with their dad as they don''t agree with his and my ex best friends actions. If our youngest is picking up on their negative vibes what can I do to make sure she maintains her relationship with her Dad?? Or is that down to him to reassure her?
Thanks in advance for your valued opinions.
Bulldog

Have you read the article on wiki called Annies story the transition bridge. It will explain whats happening here and how its both parents responsibility.

Thank you MrsMaths for pointing me to that article. A very interesting read although I must add that when my daughter goes to her Dads, I am not unhappy about it at all. So can we be clear that My next question is not going to be "shall I stop my daughter from seeing her Dad cos it''s upsetting her". I spoke to my ex last night and said it was up to him if he wanted to bring her home as its on his time. I have great relationship with my Dad and I want to give my children the same opportunity.
Bulldog

It wasnt meant as a criticism nor did I think you would stop contact but I hoped it would help you understand your daughter.

Could you recommend dad read it so it might help him too?

Think the article demonstrates the need for teamwork and understanding.

Well done for wanting your daughter to have a good relationship with her dad. Working together is the best way forward, if that is possible for you both.

Mrs Maths, my ex would scoff if I suggested he read an article and I suspect he would sound like "Jim Royal" and say "article my a@@e lol.

Stepper, I can only hope that my daughters have the same relationship with their Dad that I have with mine. But I doubt it. He called me a poisonous bar steward in front of our kids last week. He blames me for the older ones not wanting to see him.
Bulldog

Think all you can do is continue to encourage daughter to see dad and see if you can encourage dad to use distraction tactics to " discourage" the stomach ache leading to bringing daughter home.

We went through a bit of this with my husbands youngest who has attachment issues due to mums insecurities, (mum always has to tell the children how much she will miss them and how unhappy she is whilst they are at dads, shame she didnt think about that before she had an affair!!) we found by ignoring the illnesses by using distractions, the child is now less anxious now.