Missed period then LONG period

I missed my period for almost 2 months. Then I got it and it was extremely heavy for 14 days. I stopped bleeding one day and started back heavy bleeding again. In total I have been bleeding for 20 days. I’m making an appointment on Monday. I just want to see is anyone else has experienced this or if anyone has a clue as to what it may be. I went to the dr when I missed and they done a pregnancy test (neg) and a pap and everything came back great. I’m very worried now that it may be something very bad 😭