I've had something similar happen. Turned out it was a mass of ingrown hairs, all about 1 & 1/2 inches long, tightly grouped. It seems the mass grew together as a big clump, almost looking like some sort of ropey/cable looking string of hair, very uniform, connected by a puss filled sac/bubble at one end.
One night I noticed I had blood dried around my belly button, and a very, very foul smell. Must've been infected quite badly, and I had never noticed. The infection mustve burst and drained while I slept, and I awoke to the dried blood around my belly button. The inner portion of my belly button had fluid in it.
The odor was very distinct, and quite pungent. Even now, the smells persists despite bathing regularly, cleaning the wound multiple times, trying rubbing alcohol/hydrogen peroxide. Tried a topical anti bacterial cream and a swab. Nothing seems to be able to kill off the odor for good, but it isn't quite as bad as it initially was. I'm not exactly sure what the problem might still be, if the infection is lingering or if perhaps it's a fungal/bacterial infection requiring some kind of anti fungal/antibiotic treatment.
Sorry if this grosses you out, but I like to be thorough, and figured as much info as possible might lead to others with similar experiences being able to help me.