What Causes Missed Periods, Chills And Abdominal Cramping?

it has been 3 months since my daughter who is 13 has had her period. I know for a fact she is not sexually active but she weights almost 300 lbs. She will occasionally have cramps but a period doesn't show up. She puts a lot of stress on herself with her grades-she thinks a A- is a bad grade so she stresses a lot. Is this a problem I should go to a gyn. with or try and get her to chill out a little or is something major going on?