if you didnt notice it when you werent pregnant than it probably happened in the morning which would mean that the estrogen surge would be gone by the next morning...if you check out my charts you will see i have a dip pretty much every time...also in the chart gallery youll notice just about everyone has a dip almost every month..even if its just a small dip.....
also....heres some useful info
It is not known where the "Implantation Dip Myth" got started, but it is one hard myth to break. Women have grown to believe it means that implantation has taken place when their temperature suddenly drops sometime after ovulation, and then goes back up. There is no scientific basis for why implantation of an embryo would cause basal temperature to drop. In fact, implantation of an embryo may cause temperatures to slowly rise (see Triphasic Temperature Pattern). There is however, a reason for what is called a "fallback temperature."
If you'll focus on the "plasma hormonal concentration" section of the chart you can see the relative levels of progesterone and estrogen. Notice that a time after ovulation that estrogen levels surpass progesterone. This is the cause for the fallback temperature. The hormonal chart may be a little misleading, fallback temperatures can happen anywhere from 2-9dpo and do not mean you are or are not pregnant. Do not give up hope if you have a drop in temperature before 12dpo, it doesn't necessarily mean you are not pregnant. It is not a pregnancy symptom though.