What Causes A Missed Period With Negative HPT?

I am a healthy 24 yr old woman. My husband and I decided to TTC. I stopped taking BCP at the end of Jan. The first day of my last period was Jan 17th. I am currently roughly 2 weeks late. I did a HPT about 3 days after my missed period and it was neg. I had foot surgery the 25th of Feb and they also did a urine test, which also came back neg. Is there any chance at all I could still be preg. even with 2 negative HPT? This is unusual for my cycle. Thanks!