Good afternoon Retro.
Nothing serious or otherwise springs to mind on the history that you have give.
Blood tests being normal rule out a leukaemia which the enlarged mesenteric lymph glands might have suggested.
I am not sure how a MRI of her brain (head) would help, but the Drs may know something that we don't.
Mesenteric adenitis might well explain some of her symptoms and this is the result of a recent infection and are of little danger, not requiring treatment.
i look forward to the results and your feedback, but from what you have described and the negative results, it doesn't sound a big deal.