If you keep it, make a note of illness, and go on charting i think FF will discard it once your coverline has been established (i.e. when it drops a little but not back as low as pre-O in a few days). All in all i wouldn't worry about it, it means you don't know if you O'd yesterday or today, but do you really feel like BDing right now anyway?

Rest up, look after yourself and see what tomorrow's thermometer says.

Bx