DD came and chatted on my bed late last night. It all came out:
She's been using MyFitness App to monitor calorie consumption and has done all the extra walking/exercise deliberately.she said she's deleted them now
She's been using another calorie counter app to restrict her calories to less than 1000 a day.
She watches what I eat and copies.--I told her I'm 48 and have a slower metabolism
She admitted she feels tired all the time even climbing the stairs at school 
Her friends have noticed her hands go purple when she's cold.
She's noticed her bones showing on her chest, and doesn't like it.
When I told her the other day that she should be having at least 2000 calories a day to just live let alone exercise she was surprised, she didn't know.
I'm heartened that she's telling me, perhaps she's getting the message. She said she was really going to try to eat more and today had granola, strawberries and a piece of toast with Flora on it. She's just home and taken some left over couscous salad and a small slice of cake as a snack.
She's embarrassed to eat more than her wrap at school infront of her friends who are all weight obsessed, so I said am I right? as long as she had a filling breakfast, after school snack and decent dinner that was ok. Also that she needs to mix with other friends.
Calmit that's horrible about your daughter's swimming costume and missing her class. So sorry.