What Causes Missed Periods,weakness And Shakiness?

Hello. My friend is experiencing some trouble. She is randomly becoming weak and shaky along with feeling sick, and she has missed her period for 2 months. She's a virgin, so she knows she's not pregnant. We're just worried. Thank you for any information. She's 13 about 160 lbs and has no medical history, as her parents don't take her to a doctor.