Hi,
Well, from my experience, bc pills and injections can sometimes cause irregular bleeding and make your periods a bit screwy. When that happens you should definitely see your doctor about that.
In your case, I would be a bit suspicious about the bleeding after sex. There could be various reasons for that: it could be something as simple as lack of lubrication during intercourse, or it could be a bit more complex such as tissue damage or an irritation within your vagina? Birth control can often make the vagina more 'dry' and less lubricated during sex...
Whatever the cause is, you should seek advice and treatment from your doctor/gynecologist as soon as possible. You might also consider, not having sex until after you see your doctor for an appointment.
Take Care