What Causes Long Cycle Of The Menstrual Periods?

Hi im 23 yrs old female of course and my cycles have been very very long like anywhere between 20-28 days long its heavy bleeding for the first week and then normal bleeding the rest. My famuly has a history of cancer and alot of female members have hystorectomy's (or how ever u spell it) I am scheduled to see the doctor this friday but of course my cycle is here yet again! Now from the age of 13-18 I was on depo. I got pregnant and had my son at age 20 went back on depo for a little while and have been off of it for about a year now and I have constanyly had there extremely long cycles. Im tired all the time and have constant headaches does any one know what this could be or is it too much to TRY to narrow it down?