A camera cannot collect dust on the image sensor on its' own. It needs some help.
How often do you change your lens attached to the camera? Do you change lenses indoors or outdoors? Do you think your lenses are clean and free of dust and debris, too?
Do you have an air blower? The "Rocket Blower" brand is popular. You squeeze, and puff of air comes out that can blow away dirt and dust from an image sensor. It is also effective with lenses. Just remember to hold the camera so that any dust that gets dislodged by the blower will fall out of the camera, or a lens.