What do you think is wrong with her?

She is 12 and i think she is cutting. She has cuts up and down her arm and if we ask her she says she had a fight with the dog and the dog won. Now if there were just a couple scratches on like her wrist but these are deep and up and down her arm on both sides. She usually wears sweatshirts like even when it is like 70 degrees outside. She is accident prone so do you think that these are really accidents or is she deliberately hurting herself. Also sometimes when im in her room ill find stupid little things like bent bobby pins on the floor and if i throw them out the next time i always another one. If she is cutting what d you think i should do to support her i mean i think that she might have stopped if she was because she always has rubber bands and snaps them on her wrist. I know that if she was cutting she would have told her friends because she tells them just about everything. Like i said she is usually wearing sweatshirts but i saw these marks when she was in her soccer uniform which is shirtsleeves.