I think iv had a miscarriage (graphic pics)

I had unprotected intercourse about a month and a half ago with my partner. About 3 days ago I started having extremely painful cramps in my lower abdomen ,I thought I was going to get my period ,yesterday my "period" started with heavy blood flow. Today I went to change my tampon and I found this giant clot which is about 4 cm which has never happened to me before.
In addition a week ago I did a pregnancy test and it came up negative,so I'm not sure. Should I go see my doctor???