dust in the lens is very common with zoom lenses. Whether you have a filter or not it does not matter. When you zoom in/zoom out, there is air exchange from the outside environment to inside the lens, naturally dust (i.e. particles in the air) gets into the lens. When you look into the lens, if u see no dust does not mean have no dust, you have to use a bright torch light to shine inside and you will see all the dust. Even a brand new unopened lens may have some particles in the lens.
Dont worry about dust in your lens as it does not affect your image quality.