She sounds very similar to my own DD, especially over this Xmas period when it was just one bug after another & it floored her :( - at 7 we were in a similar situation too, no real answers bar IBS & the rest I worked out for myself as far as food intolerance goes - like yours, if mine has a long stretch of good health her lactose intolerance goes completely, if she's run down then she struggles to digest both wheat & dairy, she's also intolerant to chemical additives.
At about 8, mine started to have accidents, fractures, sprains, back to back for almost 2 years, often from the simplest of accidents, she also complained of aches & pains, headaches & exhaustion from as far back as I can remember, also at times over sensitivivity issues, with touch, sound & sometimes light, sometimes anxious too, though this was always slightly there, it got worse if she was ill, which generally became more frequent.
Like I say, no real answers for years, fobbed off as growing pains, IBS & picking up various bugs, over time I worked out how to manage it all holistically with diet & supplementing with spirulina if I was worried she wasn't eating well or it was passing straight through her, using things like manuka honey & turmeric to calm her stomach/IBS as needed -
Her IBS was mostly under control bar a bad few months when nothing seemed to work & then by chance we realised it was her preventative Asthma inhaler that was making it all worse, stopping that was pretty much an overnight cure of the ongoing bad IBS symptoms, though she did stil ave flare ups, we could usually pin point a cause, diet or stress.
Anyway, eventually we realised that DDs extra bendiness wasn't normal, ( it was to both me & DH as were were the somewhen young) & thanks to an awareness flyer doing the rounds on Facebook we suddenly realised there was something that strung all DDs symptoms together - Hypermobility Syndrome - she was finally diagnosed just before Xmas & it now all makes sense - in that she's getting older (10) & hormones play apart in it getting worse. - it's a defect in the collagen that can affect every thing from skin to stomach to joints & can be mild, or severe, or vary
- it also means she has a fibromyalgia like over sensitivity to pain - if she's ill or run down, then it's like the volume button on her pain signals turns up - IYSWIM - inbetween she can be healthy & bouncy & full of beans, but her slack joints mean her body works harder at doing al the normal stuff, so she can get over exhausted more easily, so run down & ill.
She does also get the painful lymph nodes in her stomach - Mesenteric Adenitis with every even minor illness she gets, this isn't uncommon & could well be your DDs problem alone, but in my DDs case she feels the pain of it more so, because of her hypersensitivity, it can get so bad they have us rushing her down to A&E with threats if suspected appendicitis, so it IS very painful for her & can make her feel sick, exhausted, look grey, dark circles & not want to eat, but though it looks & feels like a stomach problem is actually isn't
If you DD is flexible, good at ballet, gymnastics etc, then look into HMS, if not, then look into Mesenteric Adenitis
HTH