Okay well my sister has the same problem.
She has gained weight and being missing period on in off.
first i can say go to your doctor..because this doesnt always mean that you are pregnant, but still take a test the third time. sometimes pregnancys wont shore positive till later on your pregnancy.
your doctor will take care of yourself, in order if you would want to get pregnant later on life.
take care of your self right now, that your young. my sister is 30 so its harder for her to conceive, when now she is trying to get pregnant.
go to the doctor!!
hope this helps!
guera24