What Causes Late Period With Negative Pregnancy Test?

Im 37 yrs old.. my period are like clockwork every 25-26 days 2 days before i get water weight gain. iam now 6 days late (feb 12th). i have a heap of symptoms that i NEVER get. my last period was jan 18th but only lasted 7 hours(ish) i hav dont hpt neg but that also happened with my twins who are now 17. what else cold this be if im not pregnant? please help