Lower bachache and no period for 3 months but NOT pregnant?

im going to my doctors next week but before i do would apprecter any answers.despite having no perios for over 3 months and lower backache for about a month i have got loads of negative test including one from doctor. so what do you guys think it could be? im 20 and had periods since i was 12 and obviously nothing to do with menopause. just worrying thinking something could be really wrong. thanks :)