Why does my dog's butt smell so bad?

FIRST OFF,before you say stupid sh*t like uum why exactly were you smelling your dog’s butt? THATS NOT WHAT HAPPEND!, so stfu lol. this is actually a serious question, one of my dogs are pretty small so when he lays on my bed he lays directly on my pillow and fits perfectly and when he gets up the pillow always smells dirty, im constantly changing my pillow cases and now i keep my door closed so he cant get in,but sometimes people go in my room to put things away and just leave my door open, even though i tell them to close it. but hes not even dirty he gets bathed and one time i even took and old washcloths that nobody use and wiped his but with water and dog shampoo (that is officially his buttcloth), and his butt still smells dirty, no matter what it smells bad where ever he puts his butt, like if he is on the couch, and i lay on the couch instead of sit, i can smell @ss, and its only wherever he goes, so I KNOW its him, and its really annoying me. none of my other animals have smelly butts. could he be sick or something. and again this is a serious question so pllllleeeaase dont get smart or make fun of me, or my dog! he cant help it