First time using OPK

Karen • Pregnant after our 3rd transfer! After two failed transfers (1 fresh and 1 frozen), we did another round of egg retrieval to PGS the embryos and we did 2 ERAs. We found out we needed another day of progesterone before transferring!

I'm uncertain if I ovulated this month yet since my bbt is quite low after a spike in temperature so I used an ovulation strip which test negative. I was wondering, would the strip be positive if I have already ovulated? in other words, could the strips tell me that I have already ovulated or will it only tell me when I'm about to ovulate?