According to the symptoms you reported, there are several different diagnoses that should be included in diagnostic procedures.
The most probable is a condition similar to cataract. Cataract itself is not very probable because it usually appears in older patients and the doctor would have already seen it.
Retinopathia (disorder of the retina) might cause seeing spots. The spots appear because the cells of the retina that should receive the light are somehow damaged. This condition can be diagnosed by dilating the pupils and examining the posterior part of the eyeball including the retina.
The real cause can also, be located somewhere along the nerves and in the visual centre of the brain (the occipital lobe). Adequate neurological diagnostic procedures performed by neurologist might help you solve the problem.