I don’t know if there’s a more specific area to post this (I couldn’t find one?). I think this is going to be long so please stick with me, I just want to give the full picture and not drip feed 
I’m concerned DD might be showing early signs of anorexia. She’s 12 and in year 8. Her dad recently re-married and moved in with his wife and her daughter, who is DD’s age. They’ve never got along - they’re in the same year at school. This girl sends DD all sorts of mean messages when she’s hear but speaks in a babyish voice and is sweet to her face (although can break out into screaming/kicking, even at nearly 13). DD as a result has become increasingly reluctant to go to her dad’s - fairly sure she was faking having a migraine last weekend to avoid it, and has decided to stopp going midweek because she says it disrupts her routine. I accepted this at face value until she started confiding more about not getting on with ex-H’s DSD. I’m also pregnant (due in Feb) and DS moved out in September (to study, not because of anything dramatic), so it’s been a relatively big year for her.
She’s always been relatively slim and very sporty - trampolining and dance both in and out of school, a martial art out of school and part of the school netball team. This year she’s suddenly taken on much more dance after getting into a few companies, and has also started going to boxing twice a week before school, and going to badminton and volleyball at lunch. She has packed lunches and her lunchbox always comes back completely empty which it never used to. She’d often leave orange peel/wrappers/tinfoil/clingfilm in there. On weekends she doesn’t eat lunch - she has breakfast (sometimes as small as an orange, the past week I’ve convinced her to eat a (plain) croissant on most) but very rarely will eat her lunch. She used to always just eat what I made for younger DC and we’d sit around the table and eat it together. Over half term she asked to make her own, but I would repeatedly call her and remind her of lunch and she’d keep saying she wasn’t hungry and would wait for tea, or would get some crackers and take them up to her room. I found about 10-12 crackers in various places her room this morning, not even hidden. The only days when she ate a proper lunch were those when her friends were round and I made them the same as the whole family. She has packed lunches on weekends because she is at dance rehearsals all day, which if I’d known she was going to take up so much during the week I absolutely would not have allowed. (I let her do it on the basis that she quit her out of school trampolining and did just one session of her martial art a week). And again, the lunch box comes home totally empty - like the contents has just been disposed of.
Now onto tea: she does eat tea, but never all of it. She used to eat all her tea and have pudding. Now she often rejects pudding and just eats sort of half. For example, last night (after a day in which she’d done boxing and trampolining at school and had a dance class later in the evening) we had nachos with tomato sauce/lots of cheese, peas, and garlic bread. She ate all of the peas, I think two pieces of garlic bread (I didn’t manage to count but it was no more than two), and very much picked at her nachos - probably ate about half. When she got back from dance I offered her soup as a sort of supper (because she’d not eaten a lot of tea) and she said no. She also used to always have a snack first thing when she got home - not any more.
So. Am I being paranoid and overreacting? And if I’m not, what do I do? Who do I speak to? How do I manage it better? I’ve told her she has to quit 3 activities after Christmas (she said she had to do the whole term because she’d signed up - I’m skeptical so might ask the school), but she definitely does love dancing so much so I’m reluctant to make her cut down on that.
Sorry that this is so long, and please be gentle. This is really new and scary territory.