What Causes Abnormal Vaginal Bleeding While On Birth Control Pills?

Hi, I had my period 2 weeks ago, I have changed my pills as old one was giving my headache, and I was told to have extra protection but it just happened right next day of my period finished and second time I had sex 4 days later then first one but that time i took morning pills and next day I had x-ray for my back, and today I am bleeding again but its brownish, why? is it normal? Am I pregnant? If I am pregnant will I have a healthy child? Thank you very much in advance.