It could be nothing but could be something...hard to tell. Do call in even if its just for piece of mind.
It doesn't sound serious but I would at least call your doc its normal to spot but yea we don't know your situation so its prob best to at least call and ask about it.
i don't think you should worry unless you have cramps and heavvy bleeding..or bright red spotting...but why not go and check it out anyway?
Usually the dark blood is old blood. Do you have cramping with it? It never hurts to call in to your dr to feel reassured.