What Causes Long Menstrual Bleeding?

hello, i have been bleeding for over a month now, I thought I was having periods. Yesterday started my fourth one. I was diagnosed with pcos in 2008. I was told that I could develope uterine cancer later in life. I went to my doc and had some blood tests and ultrasounds done last Tuesday. My doctor has sent a referral to my gyn. Waiting on that, meanwhile I went and got my ultrasound results from the office and it says that my endometrium is measuring 19.3mm. I read on a credited medical site that if it measures less than 5mm there is probably no uterine cancer. Can you help me figure out what could be going on?