What's wrong with a person having these..?

My friend is a very lively and cheerful person, bu she's so pessimistic. Pessimistic not in the way that she gets easily depressed. I'll just tell you what I think of her. In facebook, I still remember who I talked to (my friends are anime role players so basically, I have at least 10 or more friends with similar names but I remember the person whom I had a conversation with) even if the conversation was last year.. She does not. I think she's just not inclined to remember the happenings if it occurred a few weeks to a month ago. I already knew her by name even when we were not close yet in FB, I remember I always see her post that she has fever.. When we became close and even became textmates, every now and then she come down with fever, she's always in and out of the hospital. But she's very lively, like, she said to me one time that her hand bled where the needle was injected coz she moves around the hospital bed a lot because of laughing from our conversation. She texts me and says, "LOL, I'll die soon so I have to be happy." "If you're God, I'll ask you, 'Why did You give me life if I'm just mean to suffer' LOL!" She's pessimist and always tells us she's gonna die soon, but she seems okay with it. She works a lot in school, president of different clubs, school assignments, projects, extra curricular activities. She said was pulled out from her section and placed in a different class, a class of 4 students who have sickness. Accdg to her, she has post traumatic stress that's why she always has fever. One "psychotic" facebook user is verbally torturing her, and us, her friends, were in defense. She says she always cries and always apologizes to us, saying it's her fault we became involved. She easily cries, she's emotional. But as I said, when she's happy, she's really happy and lively. She is somewhat slow, although she can solve riddles (in the form of murder cases). I send her English quotes, she said she doesn't understand coz it's too deep, which I don't believe. She reads Tagalog (our language) poem, she says she doesn't understand, because it's too deep, which again I don't believe. I think she just doesn't want to think seriously. The last time now, a person close to her said my friend had headache, then collapsed. She did collapse before. I don't know, I think she has both physical and psychological problem. I don't think it's Alzheimer's, that's too severe, she's 14. I don't think it's dimentia either. Everybody forgets things sometimes, but I don't know what's the problem with her. Can you please help?