Media is anything related to downloads or to your sd card. So if you haven't been downloading then your card is in use. If there have been any changes to what is on your card there may be indexing going on. If you are using any app that accesses the SD card that could explain it too.
With such little time elapsed, no reason to think it is a problem. If it continued however and with no known reason, then you could have a corrupt file or the file system itself may have become corrupted. In that case, move any apps off of it, back up your card, format it, and move your files back in batches.