I had my period nov. 3rd and I had it for 5 days. So nov. 8th it was done. But then since nov. 8th I have had this dark brown almost black discharge for 18 days. I heard of implantation bleeding but I haven't had sex this month. I have been with one person when having sex. So it can not be a disease. Also I heard that implantation bleeding only lasts for 2 to 3 days max. Im extremely confused and I don't think I could be pregnant if I had my period already.

The timing would be unusual for implantation bleeding and the duration would be as well.
Are you on any sort of birth control? Sometimes women will have some irregular bleeding due to hormonal changes.
Some will also have some brownish discharge prior and/or after a period once in awhile.
For the peace of mind you could contact your doctor and have it checked.

Not to be mean, but just because you're having sex with one person doesn't mean it can't be a disease.

I said I wasn't trying to be mean. You did mention it after all. Anywhoo, I am happy all is well.