I hope you do get to the bottom of it for all your sakes:(
FWIW for quite a while I couldn't work out what was going on with DD either, talked & tried to get her to open up without putting words into her mouth, tried story writing, roll play games - her playing me, me playing her - her playing other kids at school, teachers etc - nothing :( , she really didn't seem to know, bar what seemed like the usual gripes over school yard squabbles & "the naughty boys" which were a common topic amongst her & her friends, but didn't seem that serious - there had been 1 incident she's told me of, but told me it had happened to her friend 
Eventually I took her back to our GP, as even though she was still eating a good diet, she was nibbling & constant diareah & stomach pain, not sleeping etc - she was also often - not sure how to word it - grumpy isn't quite right, but difficult, stroppy to how she usually was, but not always, just some times & always seemed really full of remorse & punishing herself before I could even tell her off - I couldn't put my finger on it, it didn't feel right, but talking with other Mums, even a child psych friend, just thought it was her age -
anyway the GP asked if she had any trouble that was making her Tummy poorly - & she suddenly just spilled it all out
mostly centred around 1 out of control boy, who was quite new, smashing up the class room etc, he had attached himself to her & she felt she had to be friends with him, to stop him hurting her teachers - LOTS more, I was gobsmacked & felt like rubbish, as she -
the Teachers weren't aware of any direct bullying, knew there were class problems but didn't realise - or didn't own up to the effect it was having on the rest of the kids - my DD was being bullied, but in a way were she felt she HAD t be friends with a boy she really hated, & it was eating her up - if she kept away from him, he came after her - it also turned out the story she told me about her friend HAD in fact happened to her & lots more, basically she was living in fear, but felt ashamed & didn't want to cause trouble :(
I'm not saying this IS whats happening with your own DD, I really hope its not - but certainly worth baring in mind that even when we have a good relationship with them, sometimes they bottle it all up & can't even bring themselves to open up to us :( - I was devastated by that, but found out its actually not that uncommon :(
The other thing that might be of interest as regards your DDs diet - &I can understand totally that ANY food is better than none when they are wasting away :(- there is a superfood supplement SPIRULINAit comes in compressed tablet form, from GNC & the likes, its totally natural & safe for kids, & is PACKED full of good stuff, vitamins, iron, enzymes for digestion AND lots of pottasium - it really helped DDs health - I still give it to her now if she's looking run down - but used to give her 3 cut up pills a day
Hope thats of some help
Good luck