OPK as pregnancy indicator

If what I'm learning about using OPKs as a pregnancy indicator because they are detecting similar hormones, would an OPK therefore ALWAYS detect it if you were weeks past your AF due date? Your levels would have to be high enough by then right? 
I ask because I had a crazy light AF last month that came 5 days early. I gave up on my chances after BFN tests at 13dpo. I've begun OPK tests this week (still all negative), but am starting to read about positive OPKs for pregnancy, and I'm having pregnancy signs. I'm out of HCGs now otherwise I'd take one! Any chance AF was IB and I could be pregnant, even though my OPKs are all negative?