Hi, If you have always had a regular period, this is cause for concern. As urine pregnancy tests are negative, chances of pregnancy are less likely. Yet, one blood test for definite exclusion is a must; a trans-vaginal sonogram is an additional benefit. Once pregnancy is excluded, you should test for the possibility of hormonal disturbances; such can occur when there is any stress, infection, sudden weight changes or use of some medications. You should see a specialist for further management. Hope this helps.