Why do I smell flowers after sneezing?
This will be a hypothetical answer for the moment, so take it with a grain of salt until I can find sources or someone else does:

My best (educated) guess would be that the smell is there all the time, and when you sneeze you reset what's currently being "ignored" (in that you are not aware of it, though it is being sensed) by your brain momentarily. The source of the smell may be up in the air - it could be cologne/perfume, fabric softener residue on your clothes, what you ate for breakfast and are currently digesting, the bar of soap in the kitchen, etc. - but I haven't heard of "scent illusions" before.

One very quirky way of possibly testing that out is to remove yourself from your regular environment, sniff some ground coffee beans for half a minute, and then re-enter your home or smell your clothes to see if any of the scents match up with what you do smell when you sneeze.

My apologies that this isn't a thoroughly-sourced answer, but hopefully it acts as a good starting point.