Irregular periods can have many different causes. I will try to go through some of the more common causes of this phenomenon that may give you some idea of what might be going on. The best types of physicians to see for this issue are a primary care physician
such as a family doctor
or internal medicine doctor
or an OB / GYN.
Irregular periods are usually caused by anovulation (cycles where you do not ovulate an egg). One disorder that causes this is Polycystic ovarian syndrome (PCOS). This is more of a metabolic syndrome which includes diabetes, irregular periods and abnormal hair growth. Pelvic pain can also be associated ovarian cysts, which is a part of the syndrome. A bruised pelvis may cause some abnormal bleeding, but would not cause irregular periods.
I suggest that you schedule an appointment with your OB / GYN or primary care physician, whoever you are most comfortable seeing for this issue. He or she can take a more detailed history of your periods (make sure to write down dates of starts and stops of your periods) and pelvic symptoms and perform a thorough physical exam. Hopefully your doctor will be able to get to the bottom of your pelvic pain and irregular periods. Good luck.