My 8 year old daughter is suddenly having trouble sleeping?

She has always been a very good sleeper.  However, for the past 3 weeks she has been having trouble sleeping at night. She doesn't seem to want to sleep in her bed and she's waking up throughout the night. She comes into my room at least 2-3 times in the middle of the night.  I've asked if she's afraid of something or is having bad dreams but she says no.  I have asked what's making her wake up or what is wrong and she just says she doesn't know. We've asked if she's happy at school/home and she says yes. Sometimes she complains of a stomach ache, but I don't know if that's just her waying of trying to get me to let her sleep with me or let her sleep in TV room (right outside my bedroom).  Also, her hair is noticeably thinner (not balding or anything but just thinner).  I'm very concerned.  Any ideas?  Is this normal for a child her age?