What Causes Short Period And Nausea While HPT Is Negative?

I am trying to conceive. My periods are very regular. My last cycle started February 16th ended Feb 20th. I thought i started my cycle early on March 11th, but it only lasted 2 days. I ve been extremely nauseated but pregnancy test came up negative. Could i be pregnant? Why would my cycle come 4 days early then only lasts 2 days?