Why are my gums bleeding?

So I have a palate expander and braces on right now and today when I was brushing, only one little spot was bleeding, not just tiny drops but more amounts of it. I brush everyday and use mouth wash and I'm still getting use to flossing but I don't floss much cause I'm still learning how to floss with all this metal in my mouth. Can someone here tell me easy ways to floss and how to stop this bleeding ( I went to dentist last month and I have no cavaties and they said nothings wrong just need to clean more ). Please help, thanks!