Ping pong ball sized fluid sac on top of testicle 2 weeks after hydrocelectomy, what is it? is this the tunica vaginalis refilling? can something besides the tunica vaginalis fill up? it lights up with flashlight test?

Maybe the hydrocele was not completely removed and a remnant has re-filled with fluid. Contact the doctor who did the surgery.

In Hydrocelectomy the tunica vaginalis sac is removed or everted, so 'it' is not refilling. It is possibly a small seroma ( collection of serum) and is almost always reabsorbed over time. In your case pl see your Urologist for follow up care who would examine and do the needful