Bleeding like this on day 19-20, heavy enough to soak through pads, is most likely withdrawal bleeding from the i-pill, which is a known effect. The i-pill also shifts your usual ovulation timing, delaying it or sometimes bringing it forward, so trying to calculate your exact fertile window this cycle isn't reliable.
Next Steps
A home urine
pregnancy test is worth taking now, since heavy bleeding needs causes like miscarriage or ectopic pregnancy ruled out whenever pregnancy is a possibility, even though withdrawal bleeding is the likelier explanation here. If your next period doesn't arrive within one to two weeks, repeat the test.
Health Tips
Please see a gynaecologist if the bleeding gets heavier, you feel dizzy, or your next period runs more than three weeks late.