I can't mix colors and paint layer on layer

Hey this will be hard to explain since I'm new to Photoshop and English is not my native language.

I got into a mode I can't get out from. I used to paint with different colors above each other, for instance, I used one color then I switched to another on the same layer and they were mixed. Now I can't do that, in fact, the color I paint on above the other color goes below that color. 

There is also a problem that I can't paint on the background layer, it didn;t bother me since I made another layer and painted on it, but I'm afraid this problem relates to this too.