Sometimes if the lining in the uterus doesn’t detach all at once, you can still have a little bit of brownish discharge long after your period is done. The other possibility is that there is an irritation on the cervix causing slow bleeding that turns brown before it comes out. If this only happened once, you can be annoyed but ignore it. If this is happening cycle after cycle, it’s worth a trip to your healthcare provider to get checked out.
When I started taking the pill years ago, I experienced some breakthrough bleeding. It was spotty and red, but it kind of freaked me out until my doctor explained what was happening. I don’t know if you’re on the pill or if there’s another reason you may be experiencing breakthrough bleeding. I do know this, though: any unusual bleeding between periods is a reason to visit your healthcare professional. He or she can help you figure out what’s going on.
Most likely, the discharge you are seeing is blood (mixed with your ordinary "self-cleaning" discharge - giving it that brownish color) that wasn't flushed out during your period. I've had this happen before and used a liner while it worked itself out. However, if you think that something funny might be going on down there, it’s probably a good time to contact your healthcare provider to discuss the symptoms.