What Causes Delay In Periods,headache And Abdominal Pressure?

I am 5 days late for my period this month. I had an IUD removed in November and I have had my period since then. It has been like clockwork. I have taken 2 home pregnancy tests and they were both negative. I am nauseous, have headaches, and there is a steady pressure in my lower abdomen, however, I have had no cramping like I feel with my menstrual cycle. What should I do?