still no period but tests come back negative...

Okay so I took 2 pregnancy tests last week that came back negative and I was already a week late for my period... so now I'm going on 2 1/2 weeks late and still no period and the tests still have come back negative. My last period was october 24th and it lasted till October 31st. I'm running out of ideas on why I still have not had my period and the tests come back negative...... can anyone help me?