What Causes Irregular Periods With Severe Cramps?

I'm a 37 yr old female with no major medical problems, my father, who is 64, was just diagnosed with prostate cancer. I have read alot about this & feel better knowing there have been many improvements in treatments & survival rates. My mothers sister just died from breast cancer in sept '09. On my mothers side of the family my grandmother & grandfather died from cancer, and 2 of my moms aunts & 3 cousins died of cancer,( breast , ovarian, & uterine). What are my chances of developing cancer? I have had some problems recently ( the past 2 years or so) with irregular periods, horrible cramps even when I am not having my period, it is always in the same spot on the right side, it kinda feels like there is something there( if that makes sense). Before I had my 4 yr old I had never had any problems with my periods, they were always on time & never really heavy & hardly ever had any cramps. I have been to the doctor & he says that my uterus is the size of a 3 month pregnancy, & will need to get a hysterectomy, he also did an ultrasound and did not see anything. My mother had a hysterectomy when see was 29, she had fibroids. I have 2 children 9 & 4 & had a tubal 4 yrs ago, i guess I am worried that something will be missed & then it will be to late. Any suggestions? Thanks for your time. jennifer