What Causes Irregular Periods, Stomach Pain And Back Pain?

i have been missing my periods on and off for nearly 2 yrs now the last period i had was 3 months ago i have bad stamoach pain and bad back pains ive goin weight and dont seem to be myself i am 45 this year and have been to the doctors 3 times i think i may be on the change but my gp says not and ideas