Hi there!
1. This is unlikely to have causes a decreased semen volume and this could be due to psychological effects or just mere co-incidental.
2. If there has not been any trauma and there are no specific symptoms such as pain/ discomfort, a medical evaluation may not be required.
3. I would assume this to be due to psychological causes and would suggest avoiding stress for the time being. If this does not seem to help, you may consider an evaluation by a urologist.
Hope this helps.
Take care!
I would sure like some help here...