Sudden mild dull ache in right testicle. was doing nothing but sitting down. is this testicular torsion?

Not likely.: Most dull aches in the testicle are just that. Most serious pathology in the testicle becomes progressively uncomfortable. You will know it when you have it.

Torsion: Testicular torsion pain is usually a severe acute pain that persists as long as the torsion lasts. Your pain sounds more like a pressure or squeezing of your testicle by your legs or underwear or tight pants. If it persists you should have it checked.

Family Practice: Testiclar torsion is considered an urgent diagnosis otherwise you may loose the use of the testicle . However what probably happened was rubbing the area while sitting down. It is sensitive. It is more common with bike riders.

Could B intermittent: Torsion. You can test for this by twisting the testicle like "coding a book", i.e. Clockwise for a right testicle, until it becomes painful. Resulting pain would have to be identical to consider intermittent torsion. Pain could also be due to spermatic venous congestion which is benign, or an epididymal cyst.