What are some causes of a missed period other than pregnancy?

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I just found out my 15 yr old daughter is having sex. Her period is 2-3wks late her breasts hurt and she has felt nausea. The home pregnancy test was negative. Any ideas of what else could be wrong?

Her period is probably just late.
First of all you should search this on Google and it will give you thorough answers. But stress can be a huge factor, it may not just delay, but eliminate a monthly period. It could also be her body just going through changes, or she could just be irregular. It could also be drugs or weight loss/weight gain. If she is still really early in the pregnancy, the test may have just not caught it. You can try this site or find another helpful one for yourself.
Like others have said, it could be stress, poor health and many other things. I would still take another preg. test again and make sure shes not pregnant cuz those are classic signs of pregnancy. Unless shes on birth control.
stress that she could be pregnant would make her late,,,,,,,,,,, i promise u