Almost certainly dust on the sensor rather than the lens. Check your exposure settings, I suspect that you will find that with the high light levels, your auto exposure will have selected a small lens aperture setting (f.16, f.22 or higher), the consequences of which are to accentuate dust spots. Wider apertures (f.5.6, f.8 etc) will probably not cause the dust spots to show. The problem is that you need to set a slow shutter speed to get prop blur (1/320, 1/250, etc), which forces a smaller aperture. When light conditions are so bright, you need to set the ISO to a low figure, say 100.