I had a small dip in temperature the day fertility friend said that I ovulated. It wasn't anywhere near as sharp a drop as yours is though.
It could mean impending ovulation or it could have just been an error with the thermometer. Did you do anything different this morning when you took that temp (talk, breathe through your mouth, etc?)
And I never received a positive with opk's either. It wasn't until I started using the Clear Blue Fertility Monitor that I ever received a positive test.
Good luck and will be interesting to see your temp tomorrow!