I think it's pretty common for that to happen, as a result of residual progesterone left in your body. My temps never drop neatly before AF like the books say they should. They do continue a downward trend as I get toward O, so it doesn't seem to be a problem. HTH.
Trish
TTC#3 c2 cd17 6dpo
http://circles2.fertilityfriend.com/home/3025b