What can stop a period besides pregnancy?

I'm going to the doctor next tuesday but I want to know what can stop my period besides pregnancy because I've taken 3 pregnancy test. Two on the day that I missed my period and today in the morning. Any answers would be very helpful.
Not currently on birth control and no more stress then usual.

Things that can cause you to miss a period:

stress
extreme weight loss
pergnancy
if your a hardcore athlete

Also, your body needs a few days to register a pregnancy. Depending on when you had sex wait a week or so later and then a test should tell if you are pregnant.

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