I finished my period around 9 or 10 days ago... and suddenly I am getting bad abdominal cramps and lower back pain and getting brown discharge and partly black. Has anyone had this before? I’ve never had anything like this over a week after my period finished!

I had sex last night and a few hours later when I peed there was discharge in the toilet and a black clot. I slept and when I woke up my pants had a light colour of brown in them but nothing too major. Then I decided I would wear a panty liner in the day in case anything else happened and when I went to the toilet I saw this!

Any help or advise would be really appreciated:)