After discontinuing hormonal contraceptives, your body needs to find its own rhythm again, and this takes different amounts of time for every woman.
You can use trackle right away after you stop using the contraceptives. However, trackle needs a complete cycle (meaning from the first day of menstruation to the first day of the next menstruation, with an ovulation measured and recognised by trackle in between) to reliably evaluate your data (and to prepare a prognosis for the next cycle).
If you have bleeding after you stop taking the contraceptive (withdrawal bleeding), you can enter this in the trackle app as your first menstruation.