Clear Blue Digital OPK

So, for the 2nd month my OPK is weird.... My cycles are a bit irregular since stopping BC, so I start OPK right after AF, I usually ovulate around day 13, but for the past 2 months I've had 1 day low with my OPK around day 4, then high in the morning of day 5 and peak at night on day 5.... Is it my cycle that's off, or is it my digital OPK??