FF says it better than me......
Fallback rise: Temperatures may show a "fallback rise" pattern. In a fallback rise pattern the temperature rises significantly and abruptly as you would expect, but then it drops again immediately before it rises again and is sustained throughout the luteal phase.
I'm not sure what causes it, but it's quite common. Most charts I've seen with an implantation dip at 5dpo go on to have anothter dip around 7 or 9dpo, so whether it's really a implantation dip or implantation dip at 5dpo when pregnant is hard to tell. I will keep my fingers crossed for you either way!!!
Best of luck