Why does my dog stinks horribly & she scratches non stop she DOESNT have fleas. she crawls 2 scratch her belly?

im not sure what to do. we have used conditionar, dawn, and dog shampoo. she crawls on her belly to scratch herself. we have carpet. she was like this when we got her. she is a fila mastiff. she has not past it on to my bull mastiff & i dont think she can. the people i got her from i think mistreated her and left her outside. we are using a different dog food and different shampoo. things arent clearing up. will this ever go away? she is stinkin up my house as much as i love her this gets annoying. she acts normal nothing else wrong just scratching non stop and loosing hair. i thought maybe it was mites but its not. i dont see any type of bugs on her at all. what could be causing this and how can i possibly fix it? thanks