There are several reasons why you may be experiencing this. You might want to make absolutely sure that you don't have a gas leak in your home for safety reasons. If that is not the case, any infection or blockage in your nose can cause you to smell something that is not there. I would start with a physical examination from your primary care physician first who might then refer you to an ENT if a problem is found.
The problem is called phantosmia, and there's several causes, but you should see an ENT to get properly diagnosed.