Rapper Nelly was detained last night in Texas while police officers searched his tour bus -- and found HEROIN in addition to a loaded gun ... and MASSIVE amounts of weed ... law enforcement sources tell TMZ. According to law enforcement sources, Nelly's bus was stopped at a border patrol checkpoint in Sierra Blanca -- and a police canine detected the presence of drugs. We're told cops pulled the bus over -- and after performing a search, they found 36 small baggies of heroin (totaling .64 ounces) and over TEN POUNDS of marijuana inside a large green duffel bag (an amount roughly the size of a large bed pillow). Cops also say they found a loaded .45 caliber pistol. (TMZ)