I have vision loss every time I cough or blow my nose. Is this a sign of something serious?

Cough. This is not common. Coughing and blowing the nose are valsalva maneuvers. These should not cause vision loss. Check with your local ophthalmologist and primary doctor on this one.
Could be. When you cough or blow your nose the pressure increases in the blood vessels. If you have an aneurysm or other blood vessel abnormality or even increased intracranial pressure this could be evident with tranisent vision loss. It is important to be checked out by an ophthalmologist soon.