Concerned about my daughter?

[Eryss]

My 17 year old daughter deleted her facebook and her twitter. She cleared out her phone contacts and only talks to/texts family. If a friend texts her, she won't text back. I asked her why and she said "I need to start early so I don't feel bad later." I didn't know what that meant and I was confused but she wouldn't tell me what she meant because she said I'd get mad.
I talked to her father and he's extremely worried because this is unlike her. She's quiet and doesn't talk much but we thought she loved facebook. She told us that no one talked to her and every time she tried to start a conversation they'd log off and we believe her. She's always been the weird kid at school who can't make friends. The friends she makes turn on her and embarrass her in front of everyone.
She told me she doesn't need friends because they'll distract her from her school work. I asked if she could talk to her friends and still get stuff done and she said "No, no, no Mommy, that won't work."
Sometimes we have conversations about school and how she's doing, she's usually doing great and getting wonderful grades. And I'll ask her everyday how school went, she'll say it went fine, then she goes and does her homework. I went to tell her to come down for dinner and I saw her on the bed with her books and papers around her and she was just crying. She was biting down on her arm so she wouldn't make noise/sob. I asked her later if everything was alright and she smiled and said yeah so I told her I saw her crying and she said she dropped one of her books on her hand.
One of her teachers called and said that our daughter may be a cutter so I went to her bathroom to look for any loose blades. I saw a lot of cover-up and neosporin under her cabinet along with small washcloths that she never uses and two band-aids. Her razor was still in her shower intact. I looked in her room and didn't find anything but I have seen a little green bag in her backpack that she never touches. I asked her about it and she said it was empty so I said I'd throw it away for her and she said "I'll throw it away...I'm old enough to throw away my own stuff, Mom."

What's wrong with my baby girl? I want to help her but she always tells me and her dad that every thing's fine even though we know it's not. Please help.