Hi Pene -Like you, I tend not to have one niche, but over time, because I have concerns about the earth's environmental issues, I have found myself writing about them and my persoal response to them.
I have a feeling I have read a few of yours with a similar thrust. Yes/no?
As for your medical stories, I love them.
To be honest, I think time will be your direction. If you don't have just one niche, you will have a few. It will happen.
Oh and the other thing...sometimes another writer will ask a question and you think...I should write about that - NICHE! I have only just finished responding to such a question, in an article.