Tri-phasic BBT and negative HPT

It was quite a blast (in the negative sense) to learn that a tri-phasic
BBT doesn't imply pregnancy. I've read TCYOF cover to cover, and it
doesn't say anywhere explicitly that this happens or why. I ovulated on
CD17, had a second jump in my BBT on CD26, it lasted for four days well
above (~0.3 degrees) my post-ovulation level. I tested on CD30, it was
negative. The next day BBT dropped to post-ovulation level, and AF showed
up in the evening. The day after that, BBT was back to base level.

Already before this 4 day period I felt very pregnant, and remarkably I
did not have *any* PMS symptoms this cycle. Also, I have never before seen
a tri-phasic pattern, and have very regular cycles of ~28 days, I've never
been more than 2 days late without being pregnant. Of course, it was a
dissapointment, to say the least, but I am pretty much OK now. Does anyone
have a clue about what happened? An implantation error? Early m/c? TIA!

--
-- I
mommy to DS (17m)
mommy to a tiny angel (Oct 2003)
guardian of DH (33)
TTC #2.5
War doesn't decide who's right, only who's left