A bad-smelling wound of any kind requires medical attention. A foul odor is often a sign of infection, and you should seek treatment so that it does not spread and become a very serious matter.
My husband has cystic acne, and they appear all over his back at times, he has even had one exactly where you are desctibing. They all have the same general signs. a bump, not red or sore, seems like something is under his skin. a marble.occasionally leaking a small amount of a fluid so foul smelling i can smell it across the room, and whatever it gets on smells TERRIBLE, until I clean it with alcohol.
I have to move it around, as if I am trying to break up something inside of the bump (I am) They dont have any obvious entry space. no hole. Until I search with a magnifying glass, and find a certain hole/pore, and I can then pierce it with a needle. It first comes out as hard white nodules, then the VERY foul smelling liquid comes out, sometimes I have to medicate them to get them to go away. One thing they all had in common too, was that before I started treating them, back when we didnt know what it was... he would always get these foul-smelling spots on his clothes. I mean REALLY foul smelling. Like an infected butt crack. sometimes it would just leak out, sometimes it needed pressure. But it wasnt until I was able to get the solid plug of whatever white stuff that was out, that I was eventually able to get all of the foul smelling fluid out, and get the lumps to go down, and go away. He still gets them sometimes, but I arm myself with 90% alcohol, rubber gloves, and a needle. After a few treatments, they go away.
NOT SAYING TO POKE THEM WITH A NEEDLE FOR YOU. it may be something different. It wasnt until I learned it was cystic acne that I dared start stabbing it. Talk to a dermatologist - either the OP or anyone stumbling on this. It could very well be a cystic acne spot. Pretty much a deep pore infection. So deep your skin doesnt feel irritated or get red. It just raises up sometimes. Not always. But that foul smell is always present.