Preg symptoms but all tests Neg. Nuva Ring?

I haven't had my period since January 14th.
The condom slipped off and I took Plan B three hours later on Jan 19th.

I bled for a week and a half afterwards.
Starting on Feb 14th, I began taking HPTs.
From then until March 6th I took one every other day using morning urine and all of them were negative but still no period.

I had a blood test done and it was negative too.
Next weekend I'm getting another one.

I restarted by birth control (Nuva Ring) as prescribed by my doctor.
Now I fell nauseous and bloated all the time. My breasts have felt kinda tender and I've been having cramps every day since I started it.

Is is possible that the hormone just isn't produced by my body?
How inaccurate are blood tests?