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I hope you know that the medicine you are taking (for whatever reason) is an oral contraceptive pill. These medicines prevent ovulation, so no period of ovulation.
Now let us consider, as if you are not taking medicine, and have irregular cycles. Calculating from the first day of your last period date add 30/32 (menstrual cycle length) days and mark the date of expected next period (the first day). Generally, the rule of thumb says ovulation occurs 14 days or 2 weeks before the next expected first day of the period. The accuracy increases, if you take an average of your last three cycle lengths.
To identify the exact ovulation date (preferably in case of irregular periods), you may use ovulation check kit available in the market between two days before and after the expected date of next ovulation.