The test that saw Ogawa test positive was taken on December 5th, whilst a December 9th one came back as negative. The fighter himself is unsure how it was in his body, and that he hadn't intentionally ingested anything but it appears that it's put down to medication.
According to one Japanese source one possibility for the positive test could have been medication Ogawa was using to treat atopic dermatitis, or eczema. Thed medication had been used by Ogawa previously and hadn't popped up on previous tests, including an test by the WBC in July.
At the moment it's not totally clear if the medication was brought to the attention of the Nevada State Atheletic Commission, but Ogawa has stated that he's willing to work with the NSAC and comply with their investigation to get to the bottom of the positive test.
If he is found guilty then it would be among the most significant cases of drugs in Japanese sport.