How to fill in a brush stroke in Photoshop

I really need help here. I am a new artist and I really found my stride with the brush tool. I use to be able to draw with the brush tool and then by using the fill tool it would automatically fill in my brush tool design leaving the background unchanged. Now when I use the brush tool and complete a shape like a circle. And then use the fill tool, the entire document gets filled in and not just the brush design. I have no idea what happened to make this change on me all of a sudden but there is nothing online to help me. It makes me really sad I might not be able to use this program like I want anymore. Does anyone have a solution to this problem.

I attached one of my deisgns which I made using the feature I am talking about.

Please please please help me.