It’s ovulation
It could be traces of HCG. To confirm, test over the course of a week and if you consistently test positive on OPKs then you have traces of HCG <25 in your system and body is taking time to lower it. It's a good thing if you're testing negative on HCG strips. It indicates that there are just few traces. Mine never really dropped to 0 before my next ovulation/conception.
If your pregnancy tests are negative then this is ovulation. I had a MVA in July, and ovulated around 20 days later.