CD 39, 12 DPO, pos OPK yesterday, neg OPK today, BFN HPT?

I had EWCM on June 11. Estimate June 12 O. I still have no AF signs, and no BFP. Bought an OPK recently (for next month) and used one yesterday, easily positive. Today? Barely a test line. Most recent HPT was this morning, FMU. Not even a hint of a line. What would cause such a drastic shift in OPK results this late in my cycle? Could I be pregnant anyway?