I have this brown bump at the top of my buttocks where the cheeks meet.

[iPhone_user]

I have this brown bump on my upper buttock where the cheeks meet, it doesn’t hurt and isn’t swollen but leaks a bad smelling liquid near it, I believe it a pilonidal cyst, it leaves red bad smelling marks on my underwear I have had it for some time like 7 months because it only leaks the bad smelling liquid no pain just a little discomfort when laying down on my back.

[swamplilly]

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.

[CurfewX]

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.