What Causes Abdominal Pain While On Mercilon?

Hello, I have been taking Mercilon combined oral contraceptives for a little more than a week at precisely the same time every day, with no delays or forgotten pills. It is the first time I take birth control pills and I have been experiencing mild abdominal pain and spots of brown discharge (which i m assuming is breakthrough bleeding, but it definitely looks like old blood). I will begin my sexual activity in about three weeks, and I m concerned that perhaps these are signs of faulty protection, especially since Mercilon has a very low dose of hormones. I am 164 cm tall and weigh 57 kg, so I would think that the dosage should be alright for my weight. However, should I be worried that I might become pregnant and schedule a doctor s appointment to request a different pill? Thank you in advance.