I would have to say that this is not really normal, but also not that concerning it if happened just one time. Nonetheless I would suggest that you schedule an appointment with your primary care physician
or OBGYN. Obviously, most women only have periods after the hormonal of the birth-control finishes. It is possible that you started your period in the way that you normally would have (if you weren't on birth control) and your body just hadn't started reacting to the birth control hormones. How I would approach this if a patient came to my office with this complaint is that I would have them start the next set of pills and just go ahead and see if this happens again. If it does, then you will need further investigation into why you're having bleeding
in between your periods. The other possibility is that if you missed a pill, or took a pill at a different time of the day then you usually do, and this could explain why you had a period in the middle of your cycle. To further discuss this problem, I would suggest that you schedule an appointment with your primary care physician. This could also be an OB/GYN, if that's who you go to see for this matter. If this happens a second time, and your doctor
will likely want to perform a pelvic exam.