What Causes Missed Period While HPT Is Negative?

I ve been off the pill for 6 months now and have had a very regular period up until recently. My last period was the third week of January (which also was later than I would normally get it.) Its now March 9th. I was a week late in January, skipped February and still do not have my period. I ve taken urine tests that have all been negative- Could I still be pregnant? What could be wrong?