Its been 36 days since i had a period, but im usually regular. tested a few days ago and it was negative. back hurts and im constipated. pregnant?

Only a week late: Your cycle may be more irregular than you appreciate & you may have actually ovulated later ... period will come or repeat pregnancy test will be positive. Recent travel, excessive under or over-weight also contribute to menstrual irregularity. Make sure the pregnancy test you use has high sensitivity & use first morning urine. If another week passes without period or pregnancy, contact your MD.