What Causes Amenorrhea?

Hello, im 20 years old.. My normal weight is about 123 pounds, im 5'4 and there's something going on with my body..i lived in argentina for 4 years and i just came back to the US on may 13.. I had my period the 6th of june, the 13 of july and it just stopped. Its december already and no sings of my menstrual , on november i thought it finally came but it was just the brown thing which lasted that same day.. I've loose weight , i dont eat much..i eat 1 time in the morning and at night i eat a candy bar and that's it. Im worried and im just afraid to go to the hospital..