Not necessarily. For some women, the temps will gradually go down, starting a few days before their period. It could also be one of those random dips. Maybe it'll go back up again tomorrow :-) Did you see a clear bi-phasic curve? If so, then you did ovulate.
My temps go down about 2 days before AF; that's how I know that it's coming.
I don't think temps before AF have anything to do with ovulation. The important thing is that your temps go up and stay up for about 10-14 days after O. If you doubt you ovulated you should do a blood test to check.
I found this info on line:
The average BBT is 97.48F plus or minus 0.25 (36.37 ± 0.12C) during the phase prior to ovulation (follicular phase) and 98.09F plus or minus 0.22 (36.72 ± 0.12C) in the phase following ovulation (luteal phase). If conception occurs, the temperature
Hi again! I agree with Helen, as long as you have lower temperatures before o and it rises and stay higher after o, you're fine! Your temps might be a bit lower than the norm, but it sounds like you have a bi-phasic curve, and that's what counts!