Thanks four your query to HCM.
The possibilities are
1. Infection of the sperm collecting tubes/ testes.
2. Tumour of the testes
3. Twist of the spermatic cord
( in which case the pain will be very severe and hence unlikely).
You need focussed history, physical examination, urine analysis and possibly ultrasound
scan of the testes.
Since all these have been done and no cause found try out the following --- Tight undergarments to support the scrotum
, ice application to the scrotum thrice daily for one week for about 15 minutes each session.
Doing Regular walk, yoga
would help. Try to avoid prolonged seating in hard chairs.
Hope i am of some help.