My period is coming out dark brown like I just finished my period but I started yesterday

Periods can change due to diet, pregnancy and the type of contraception used. If you're on the contraceptive pill, it can often come out darker to begin with (The blood darkens as it gets older, so when on the pill it's waiting to be flushed out)

It's clear you aren't pregnant, I don't think you should worry.

X Dani