I had this about 2-3 months after I had Batflu. Lasted about 2 weeks. Smelled phantom sewage.
I spoke to a nurse who completely lost her sense of smell for 9 months after having coronavirus. She was given these 4 small tubs to smell throughout the day- orange was one of them and I can’t remember the others. She did it every day and it triggered something and now she can smell again. Maybe something like that can ‘right’ the smells to the right thing.