Testicular mass in men less than 40 years old is worrisome for cancer. However, these lumps usually get bigger & bigger, rather than disappearing & recurring.
I am curious that you didn't mention pain, so I suspect that these lumps don't hurt. However, the epididymal structure that sits atop the testes can sometimes become infected. But usually in this instance, it's rather painful.
Of course, this also begs the question of whether you're describing a lump in the testicle, in the scrotum or in surrounding structures. Scrotal skin has hair follicles which may become plugged and/or infected, especially if you shave. Hydroceles are more likely to appear & disappear than varicoceles since the former are fluid collections rather than a mass of blood vessels as in the latter.
I'm not familiar with anything associated w/being tired (or not). We joke about blue balls from lack of sex but there's really no anatomic/physiologic consequence of which I am aware (although I'm always open to education!).
Unless you're in pain, have fever/sweats/chills, or notice the mass growing & growing, it's probably a benign structure. Best recommendation I can make is to go see your family physician when you notice this happening again.
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