ovarian mass

recently i had a cat scan done and they found a mass on my right ovary. sometimes its painful, i have an appointment at the womens health center, to further diagnose and possible surgical removal, my question is could this possibly be ovarian cancer and why is there pain, as far as i know there is only lung cancer in the family tree. i have never been pregnant im 40 yrs old what causes these masses to begin with. the doctor had said he doesnt think its malignant but he has only seen the cat scan. should i be worried? at times it does interupt my monthly cycle, 3 months with no period then i had one, it was extremely painful with cramping and heavy flowing. i have lost weight but i still have this slightly huge belly that hangs low is this from the mass? thanks for taking the time to answer my questions. after i see the doctor i could have more. thanks