What Causes Skipped Period And Nausea While HPT Is Negative?

Hello. I have been concerned lately about my health. I have never had irregular menstrual periods until recently. In Jan/Feb my period came a month late. Now, it has been two months since I have had a period at all. I have intense naseau suddenly and cramping in my stomach. I have also been extremely fatigued. I have taken many pregnancy tests and they all come back negative. Is there something else that could be causing this?