7 days late on my period. thick brown blood with blood clots this morning. excruciating pain in abdomen & lower back?

Old blood and clots: could mean either pregnancy or infection. I would make sure to check a pregnancy test, then if negative check it again 1 week later. You do not want to have a miscarriage and not know it. If everything is negative, then you have irregular bleeding from cycle and may need to see a GYN to regulate it again, especially after 3-4 months of similar bleeding.