The 24-year-old, who has been on the Black Cats' books since his 2015 switch from Celtic, joined the Robins on a season-long loan in the summer.
A combination of fitness and injury problems have resulted in Matthews getting limited minutes under manager Lee Johnson this season, and now it has been revealed that the player has struggled with discipline.
"He is fine now. It's nothing major, just a couple of little misdemeanours that could be unfortunate or could be misunderstandings," the Sunderland Echo quotes Johnson as saying. "We have to send the message out to the squad to make sure everybody is fully focused on getting a result."
Matthews was left out of the squad that faced Birmingham City on Saturday.