Sorry - this is long. I don't usually visit this section (yet) but your post caught my eye. My DSD is 11, I met her at 7/8 and she was exactly this way about her body image, and honestly this still crops up occasionally. At first I was concerned because I did not know about the growth spurts, and honestly we knew she didn't eat great at home or get nearly enough activity.
As we do not have custody and could not do some things, we did these:
1. Tried to get her active without exercising. Walks were common. She loves time with me, so I would "let" her LOL come for an early morning walk with me - but only if she kept up. Of course I had to walk slower then I normally would, however, it was not a leisurely pace. Her dad would walk with her to rent a movie and get an italian ice (we live in a big city). We got her a hula hoop. We gave her martial arts lessons (because she was interested). Now she is in swimming class at the park district and her school recently started yoga twice a week. Anything that helps her see herself in a strong, positive, 'look what I can do' light is good.
2. Whenever she would bring up eating (I am a Lifetime WW member, so the books are here) we would make sure to be clear that no food was off limits and moderation was the key. If you want a huge bowl of spaghetti - well, lets take a walk after dinner because *you will feel better* and it will help you digest. We also stressed that treats were best balanced with activity.
3. We explained the growth spurt thing (several times in fact). She also has an older cousin who was able to reinforce this (and the activity).
4. She loves helping her dad prepare meals, and I often get her input when I'm meal planning. She can have any fresh veggie or fruit almost any time she wants, though she does ask first.
I hate to say it, and I know this wasn't the best tactic, but sometimes you don't make the best decisions. When we had heard "I'm fat" way too often for a child that wasn't even close, DP showed her exactly what "fat" was. I don't know if that will come back to bite us one day, and it sure wasn't a nice way to deal, and I apologize if that offended anyone.
She also already had issues with "beauty". Even though I'm hoping for a girl myself this summer, these issues are just so much worse, and so much earlier then they were when I was growing up. Oh, I also have to say, I feel the girls mag "Discovery Girls" is really good for her. There's another called New Moon, I think, that was a little too ... crunchy? natural? for her, but seemed another good choice.
Last - another vote for bloating. Lactose intolerance can have immediate results or may take hours. It wouldn't be bad to rule it out by substituting that milk for a couple weeks as was suggested.
I hope some of this is helpful, and good luck!!