Hello, why you are worrying about pregnancy,have you not undergone permanent sterilization yet? If not ,have you not got your periods for more than one month now ? then only consider pregnancy test. but pregnancy doesn’t causes irregular periods and pain usually. If periods are irregular and if you have backache, it could be due to fibroid uterus or polycystic ovarian disease. Ultrasound scan of abdomen will help you to find the cause , visit gynecologist.