18 DPO...am I pregnant?

Mallory
My husband and I have been TTC since May. Before then I was on Micronor (progesterone only mini pill) for a year with no periods. After going off I had my first cycle exactly 28 days later. Last month it was 33 days. My OB/GYN requested a pelvic ultrasound (for medical history of cancer) on August 1st. At that appointment the tech told me I was ovulating. 
So, here we are 18 days later and no signs of a period. I've taken 3 pregnancy tests in the past week and they have all been negative. Could I still be pregnant?