Houston Cougars lineman Sedrick Williams says he's experienced "complications with my heart" after battling COVID back in July ... and now he's opting out of the 2020 season.

The 6'3", 302-pound defensive lineman -- a junior transfer from Kilgore College -- tested positive for COVID in early July and spent 14 days in quarantine.

But, Williams says he didn't make a full recovery.