Hi there VAnJL.
A couple of things could be happening here...
1: You have an area selected and need to deselect it
Do you see the tiny shape of marching ants in the image below? If you have any selections on your canvas—even as small as a pixel or two—you will only be able to draw/paint inside of the selection.

To deselect any selections on the canvas, hit command D, or go to Select > Deselect. You should then be able to paint anywhere on the canvas.

2: You have a buggy tool and need to reset it
Sometimes something simply goes wrong with a tool. When it happens, you can select the tool, click on the tool's icon in the upper left corner of the window, then click on the gear in the subsequent window that opens. From there, choose, reset tool.

3: You have a buggy tool and need to reset your Photoshop preferences
To do this, follow the directions outlined in the video below. Resetting your prefs will usually solve any strange tool behaviors.
I hope one of these methods solve your problem!