It is normal for all women to experience an irregular bleeding at least once in their life.
The most common reason is hormonal imbalance (not enough or too much of certain hormones) as a result of weight loss or gain, heavy exercise, stress, illness, or certain medications.
Pregnancy must be excluded or confirmed when an irregular bleeding occurs.
Another possible reasons could be STD, cervical erosions, or very rarely, benign tumors of the womb.
If the same bleeding occurs in another two following cycles of pills, you should consider another type of pills.
Do a pregnancy test!
Best wishes!