Start here:
https://www.nhs.uk/conditions/phantosmia/
As that link makes clear, it's usually something MINOR behind the problem but, as it's not gone away, you should get checked out by your GP, just in case there's anything more serious going on. (Back in my reaching days, our librarian had problems with phantom smells and it turned out that she'd got a brain tumour. While it's EXTREMELY UNLIKELY that you've got anything serious wrong with you, it still makes sense to consult a GP).