Almost all women will spot in between periods while on birth control, you are lucky that it did not happen sooner.
Things that can cause spotting:
Missing pills or taking them late
Antibiotics-reduce the effectiveness of pills
Saint Johns wort-an herb, reduces effectiveness
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It is not meant to substitute for medical advice provided by your physician or other medical professional.
If you have or suspect that you have a medical problem, promptly contact your physician or health care provider.