Number of days of high fertility, then low the next day before my predicted ovulation date(s)

I started testing on day 15 (35 day cycle) and saw high fertility. I tested each day up until day 20 with the same result. On day 21 I tested in the morning and my result was low, however my ‘predicted’ ovulation date(s) are not for another 2-3 days. Can I have a drop in LH before a surge?