One important question for me is are you on any type of hormonal birth control?
Hormonal birth control can cause spotting throughout the first 3-6 months you're on it.
If you are not on it, don't worry. This is very common. When having your period there is some blood that doesn't completely expel from your uterus. Days weeks months or so later it can expel and be a reddish to brown color. All this is, is old blood that didn't make its way out during your period. If it continues to be a problem, please talk to a parent or a doctor about the issue.
Hope this information was helpful!
Oh, and my period is usually pretty regular, give or take a couple of days. I've have my period for nearly four years now (so I don't think this is my period).