American boxer Claressa Shields, the undisputed world champion in three weight classes, has been suspended and is under investigation after testing positive for marijuana following her victory over heavyweight Danielle Perkins in Michigan.The Michigan Unarmed Combat Commission suspended the 29-year-old athlete from competing in the state after an 'oral fluid sample' taken after the fight on Feb. 2 showed the presence of marijuana, which is banned in competition by the World Anti-Doping Agency (WADA).