Bleeding starting on day 19 of a course meant to bring on your period is actually a good sign. It means the withdrawal bleed has already begun. Many gynaecologists are comfortable stopping at this point since the medicine has done its job, though finishing the last tablet as prescribed is also safe and won't cause harm.
Next Steps
Count yesterday, the first day of real flow and not spotting, as day 1 of your new cycle. Message your prescribing doctor to confirm whether to take tonight's last tablet or stop now, since this can depend on why it was started.
Health Tips
See your gynaecologist sooner if the bleeding becomes unusually heavy or painful, or doesn't settle into a normal period pattern over the next few days.