More than likely it is caused by the misuse of birth control. That can cause abnormal bleeding and some pregnancy symptoms (like sore breasts and nausea).
I understand that taking the pill regularly can be a bit difficult if you don't have a schedule you stick to. Maybe you should look into a different form of birth control (an iud, the patch, the shot, etc) if you don't wish to become pregnant.
Your situation doesn't sound very complicated, it sounds like you need to find a form of birth control that works for your lifestyle.