Temp spike but negative OPKs

Husband and I TTC for 4 years. Have charted before, took time off, and started again this month. I've used Wondfo OPKs before with success but this month is odd. I typically O on CD18 (which was yesterday). I had mild cramping last night. The past 4 days I've used OPKs morning, noon, and night and all are negative. Temp dropped yesterday morning and spiked this morning 1.06 degrees, possibly indicating O. Has anyone experienced O with temp and negative OPKs?