I know it's probably no comfort but in my life i have had long periods when i COULDN'T be pregnant and i still occasionally got the weirdest AF's - weird enough that i'd have been convinced it was a loss if i'd had sex at all that month! With clots, white stringy tissues in the blood (TMI i know) you name it.

Maybe the doc can do a gereral hormone-level BT to test everything, not just for early PG? That might give a clearer picture on what your body was doing this month?

Bx