What does it mean when I get a positive OPK then negative then positive again?

Hi there! Thank you for taking the time to answer my question! After suffering 2 miscarriages (after a healthy pregnancy and birth) I am having some testing done to see if something is wrong. I have to go 7 days after I ovulate to have my progesterone tested so I'm trying to pinpoint my ovulation day, I'm NOT trying to get pregnant this month. Normally I have 35ish day cycles but since I had a miscarriage a month ago and already had 1 regular period, I started testing on cycle day 9 with the Clearblue easy digital tests. It's a good thing I tested early because normally I ovulate on day 21 or so but this month I got a smiley face on cycle day 13 at 2pm and cycle day 14 at 7pm. Then cycle day 14pm was negative then today, cycle day 15 at 3pm was positive again. Is it normal to have positives, then negatives, then a positive again? What can this mean? Also, I am not on any fertility meds or anything. Once again, I'm trying to tell when I ovulate for testing purposes, not to get pregnant. Thank you for your help!