Welcome to DUST WORLD!
To check whether it is dust, clean your lens first, on both sides.
Set your aperture to F/22, focus manually to infinity, and take a picture of a white surface. Wall also can. Overexpose slightly.
Review the picture, if you see something even after your lens is clean.. It's dust for sure!
After which, go search Clubsnap on how to clean dust, various people have various methods. If you are not confident, send to your service centre for cleaning, have to pay $ usually though.