TeenHealthFX is concerned about you. There could be a number of things that are causing this irregular bleeding. It can be a side effect of some birth control, but it can also be more serious. This is definitely a problem that you should talk with your doctor about. Only they can determine what might be causing it for sure.
As far as pain with intercourse goes, we can tell you that although what you are experiencing is not uncommon, sex should be a pleasurable experience. Check out Does Sex Hurt for more information from TeenHealthFX on this subject.
Even though it may be embarrassing to talk with your doctor, these are important issues for you and your health. Doctors are accustomed to listening to all sorts of symptoms - they are trained to listen to your concerns and to try to discover what your health issues are. Telling the truth about what is going on in your body is essential in order for your doctor to help you. If you live in northern New Jersey, you can call the Adolescent/Young Adult Center For Health at 973.971.6457 for an appointment. If you live outside of Northern New Jersey you can check the Planned Parenthood website for a clinic in your area.