It seems that many guys get these aches and pains. I also had them a few months after my TC1. It's usually nothing serious, but it's good you had the ultrasound. It's possible everyone is just more aware of anything unusual in their remaining testicle.
Keep up the self exams, once a month is sufficient, but don't hesitate to call your doc if you are unsure about anything.
Right I/O 4/22/1988
Left I/O 9/17/2003
Tho' much is taken, much abides; and though we are not now that strength which in old days moved earth and heaven; that which we are, we are; one equal temper of heroic hearts, made weak by time and fate, but strong in will; to strive, to seek, to find, and not to yield.