Undecided Question

I started tutoring my best friends cousin. She’s 21 and I'm 23. Were both in university and we often study in the local library. Today I couldn’t make it to the library. So, she came over to my place. It’s been a good 11months and she never really needed my tutoring…she’s genius. However, lately she's been falling behind. We became really close over the pass months. I'm a lesbian, single and she's also single, tho not sure if she's a lesbian (however, I'm positive she is). We're not together, even though I would love that. But we have a bond deeper (more complex) then a mere friendship. Long story short…when she came over to my place she was in this really bitchy mood. Just snapping at me for the most trivial things. I didn’t understand why she was behaving this way. She’s a really sweet girl. Beautiful on the inside and beautiful (absolutely STUNNING!) on the outside. After awhile of her snapping…I was essentially thinking…wtf? And I got a bit angry. I ended up snapping at her quite a few times. Yelling: "Why are you getting mad when I try to correct you?" We were both yelling now. And she got up and chucked all her books and everything else off the table. I was so pissed; she broke some of my things. I walked over, grabbed her by the arms, perhaps a bit rough, and yelled, what was her problem? She winced and cried out, pulling away from me; then began to sob, repeatedly. “Oo-uch…I did-n’t mean to. I’m sor-ry…breaking your things.” Her eyes were huge, like she was afraid. She was trembling. And I noticed how she looked a bit pale, and very tired. I was shocked she was crying, and felt bad thinking I had hurt her. I would NEVER purposely hurt anyone…especially her! She was holding her left arm to herself. And immediately I was apologizing and coaxing her to please let me see her arm. This only made her cry more. Convincing her to sit on the sofa with me, I stroked her hair and back as she continued to sob, “sorry”, in an attempt to comfort her. She looked very vulnerable. I reached for her arm and pulled her sleeve up, as she tried to pull her arm away. And when I saw the old black&blue bruises that marred her skin…some that looked like they were done a day or two ago, I flinched. I knew these weren’t my doing…you could tell they were a bit old. And there were more bruises but not on her arms. She wouldn’t look at me when I asked who did it (what happened)…just cried harder, hiding her face in my neck, while I held her. After an hour of trying to calming her down, she told me it was “her fault” why it happened. She provoked it…I couldn't get it her to tell me who was hurting her. And I really need to know because I'm very worried! She spent the night. And I told her to stay as long as she likes. I don’t know what to do. But I care for her very much. So, I am asking you all…please, what should I do? What should I say to her?