My period 'ended' last week but it's back again.

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My period ended last week (or so I thought), was gone for a full week and then I randomly started bleeding again.  It started out as brown spotting and then it got heavier and darker, now it's kind of bright red-ish.  Might this be something serious?  I'm going to the doctor on Friday but it'd be nice to have somewhat understand what's going on.

 I'm not sexually active, if that helps.   Thanks.