Any theories on what causes flat temps?

[Mumintaz]

As if nobody realised it already, this month is a shocker! I've NEVER had such consternation or frustration with a chart.

If anybody has any theories or knowledge on what causes flat temps, I'd love to hear them.

Background info:

1) New thermometer and have "tested" alongside old one as well so no issues there.
2) OPKs for the first time this cycle. DEFINITE positive on two consecutive days. EVERY other OPK from CD1 until present has had two lines on it. Easily visible lines in all lighting and from arms' length. Getting slightly darker again now.
3) Maybe Baby registered ferning at the appropriate time and then back to "infertile" until yesterday and today when I got full ferning again. BUT...this happened last month as well although not until a little later in my LP.
4) If I change to the Research tuner it gives me an O date of CD10.

Any advice, opinions or thoughts?

Lisa
http://www.fertilityfriend.com/home/fd71c