Endometriosis or cyst?

I have been having an abnormally long period. At first they told me it was probably since I stopped taking the pill.. But after months of this they recommended having a pelvic ultrasound. I had this done last Thursday with the result of come back in four weeks for another one.. My Dr said it was probably a cyst that had bled into itself or endometriosis on my ovary... If it is the later we will be scheduling surgery when I go back... Have any of you gals had either? Just looking for info. Thanks in advance!