Nervous from reading these stories! How likely is this??

So I got my HCG beta blood test back. Results were less than 1

Reason I'm scared is because I messed up my birth control back in April and I had withdrawal sex which I know can still cause pregnancy.

I haven't had sex for about 12 weeks. Thought it might be smart to avoid sex with my anxiety the way it is.

What's the likelihood that those tests are incorrect?