The brain is, basically, a think-machine, even though it's "just" a lump of meat. The brain tries to make sense of stuff and piece everything together "logically".
Oftentimes the braim makes stuff up - your brain is very good at lying. Take for example vision - the eyes contain a relatively hole in the retina, yet you see a perfectly clear image. This is the "intended" purpose, but the core mechanism bywhich this is done is much more deeply rooted into the brain's main "function" - it's one of the core things the brain does. Its "thinking" is very malleable.
This can cause smaller "misinterpretations" of reality: Here's a personal example: when my grandfather died, I periodically saw his reflection in the front door of his house. It would be visible only for a second, and then disaopear almost immidiately. I had to be moving relatively fast for it to appear, and couldn't cause it to appear at will. 15 years later, I noticed it was actually my reflection, but since it was only visible in the exact same spot, from a certain angle, only in the evenings, with the porch lamp on and on a wood-textured PVC door, it took me that much time to piece all the puzlle pieces together and deduce the root cause. Me not having to visit his house all that often certsinly didn't help the situation.
The other is plain hallucination: Take arthritis. You have pain which is proven not to be caused by anything external. Your nerves just send the "pain signals", and you feel pain.
Additionally, sinesthesia isn't just something someone either has or doesn't, but it's a spectrum, and, all the senses are in fact connected on a quite deep level.
What you describe definately falls somewhere on this "misinterpretarion-hallucination" spectrum. Maybe there was nothing to smell, yet you felt you smelled something, caused fully by your unconscious influenced by past experiences. Or maybe there was a totally different smell that got turned into this smell, but you couldn't pick it out - as is the case with my grandfather and I.
This spectum can also be taken as the "physical-psychological" (cause) spectrum.
Maybe it's a one-off thing for you, or maybe it's a chain of conditions that'll get fullfilled again every now and then. There's most likely a logical explanation since the brain is inherently a logical machine, but chances are it's not. There are just too many variables at play as far as outside factors go.