Objects on the lens have to be fairly large and opaque before they start to show up like this in a photograph. Light from any particular point in the field of view strikes every part of the front of the lens and is then focused to a single point (hopefully) at the image sensor or film. Think about it: some telescopes have objects in the middle of the objective lens, yet there is no black blob in the middle of the field of view of the telescope, is there?
More common is that off axis light might be refracted by a transparent or semi-transparent substance on the lens or closely in front of the lens so that the light coming from a point that would normally be outside the lens' angle of view winds up in the picture. But if that were the case the same thing would likely be happening in the photos you took before and after this one, assuming your camera was pointed in the same general direction.
It doesn't look like dust on your sensor either. Dust spots are usually darker than the surrounding part of the picture, because you are not actually seeing the dust, you are seeing where the shadow of the dust is falling on the sensor. This is because most dust is actually on top of the filter stack directly in front of the sensor and why narrower apertures create darker, smaller, and more sharply defined shadows than larger apertures because as a lens is stopped down more of the light falling on the sensor is doing so at a perpendicular or near perpendicular angle.
This looks more to me like some strange type of lens flare. Flare usually manifests itself as brighter than the surrounding parts of the picture.