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I've started my period with dark blood before. Maybe not BLACK, but a brown color. I especially get this way if I'm just spotting for my period, which may be what you are doing? I sort of spot or sometimes my period will come on slowly and I'll get this darkish color blood and what you described with only a very little amount of blood. I've gotten this even more since I've started on my current birth control, but I got it before too. For me it's just a sign my period is starting, sometimes slowly. I don't know if that's useful for you, but I just wanted to share I've been through something similar.

Things like sex, a change in diet or exercise, or being ill recently can also throw off your period.

That being said though, if you are ever concerned about your period it is okay to ask a doctor about it. A gynecologist can be really helpful in telling you what is normal and what is not, and what things can affect how your cycle normally is. It is recommended to see a gynecologist after becoming sexually active anyway, but even if you weren't it's still good to make sure everything is healthy!