OK, so you just happen
to have an ultrasound machine lying around at home - and
the skill to use it? And you are 17, still at school? I'd rather like to hear an explanation for that!
OK, let me see. Testicles - and their associated equipment are
quite sensitive. And they just hang out there in a position that is relatively speaking, exposed, almost inviting injury. I'd say it is then, an important evolutionary adaptation that they are
a bit sensitive, as it were to train
you to protect them. It's probably not surprising then, that for primitive tribal people who wear minimal clothing, the most minimal piece of clothing following a belt
, is a loin cloth covering the genitals, and it is the men who wear it even before the women do. Whether or not this protects
the genitals, it no doubt serves as a reminder to be careful of them.
And incidentally, that is an important function of the foreskin - it clearly protects
the penile glans. If you have ferreted sufficiently around the 'net regarding circumcision, you will soon have encountered the "lunatic fringe" who propose some really perverse ideas that circumcision is variously practiced because of the "need" to retract (remove) the foreskin for intercourse prior to adulthood (the "male hymen"), or that because at least loincloths are worn and "we" no longer live in the grasslands, then there is "no need" to protect the penile glans.
Anyway, suffice it to say, that your testicles - and vas etc. - are tender to warn
you against doing anything which, taken to extreme, might compromise their essential function. Just as your eyes are particularly sensitive to warn you not to poke them. On the other hand, it is in fact, relatively difficult
to do actual damage, and simple squeezing is not particularly likely to be a problem. Certainly, although it is generally a "one-off" situation, when one operates on the testicles as for a hydrocoele, they can be handled quite substantially and may well be rather tender for some time following the procedure.
A particular matter which is addressed during operations (such as vasectomy) on the scrotum, is the possibility of bleeding which because all these structures are so loose
and mobile, is not restricted by pressure of adjacent parts and if not meticulously controlled by diathermy, can lead to a substantial haematoma (blood collection) afterwards. I am sure you are aware of how tender even limited bruising can be, and persistent post-operative discomfort here is a common complaint (and reason for seeking review).
It is possible
that if you handle these parts roughly, you may produce some bruising which will remain tender for a few days. Unless you cause substantial
bruising to the testicles themselves however, which is a situation that may restrict the internal blood supply and cause necrosis but which is characterised (as Jim notes) by being very
painful, then I think you can be assured that you have caused no real harm. An experienced sonographer would of course, be quite able to recognise any other abnormalities of concern.