ABIU to be seriously concerned for my daughter?

[WeAreTheOthers]

My DD turns 15 this June and I've recently become incredibly worried about her. She's always been a little chubby but has always acted as though she doesn't care about her weight. However when I checked her emails recently (I do every so often to make sure she hasn't run into any creeps, you know what it's like on the internet) I see that's she's been talking to her friend about attempting to lose weight without my knowledge. When I confronted her about it she refused to answer me, instead opting to berate me for invading her privacy. I believe she has changed email addresses now as that account has not been active for around a week but she refuses to tell me her new address. I logged on to her old address yesterday and it seems that just before she changed accounts she told her friend that she's probably only gaining weight, and wants to be able to see her ribs clearly.
In addition to this I recently found that she has gone through 3 razor blades in 3 months, and the last time she took a shower I found a small smear of blood on the edge of the tub, as well as several missing plasters. It wasn't menstrual blood as she finished her period two weeks earlier and is very regular. She wears short sleeves most of the time but never anything shorter than knee length, even in summer. She claims she doesn't have the thighs for it but I think she may be cutting her upper legs. DD is smart and if she were to cut I think it would be an area she could cover without arousing suspicion. I've also been watching her closely and she seems to give up easier than before, she used to be a fighter but it now it she seems almost resigned even though in public she still acts with the same 'f*ck you' attitude.
My mother thinks that DD is just going through puberty and is attention seeking but I am have serious concerns which DH shares. ABIU to be so worried about her? Or am I being stupid for not trusting my gut instinct and taking her to the doctor's office?

[bigdeal]

from what i know of my dds friends its the ones with the controlling , snoopy parents that are the ones cutting , smoking , meeting random blokes online and hiding it all from them so they have some control in their life , you need to back right off .