Smelling things that aren't there is called phantosmia. It can be unpleasant and affect how things taste. But it isn't usually serious and may go away by itself in a few weeks or months.
Phantosmia is usually caused by an infection such as:
Less common causes include:
Rinsing the inside of your nose with a salt water solution may help temporarily stop the strange smell.
You can make a salt water solution at home.
You don't need to use all of the solution. Make a fresh solution each day, don't re-use any left over from the day before.
Some pharmacies sell sachets you can use to make a salt water solution and devices to help you rinse your nose.
Your GP may refer you to a hospital specialist for tests to find out the cause.
Treating the cause of your phantosmia may help get rid of it.
Sometimes it might go away on its own, but it can be permanent.