Last night i masturbated but held myself from orgasm-ing. trying to stop so i thought i shouldn't do it. worried sperm went into bladder. or blue ball?

Ejaculation: You you stop yourself from ejaculating the ejaculate doesnt go into your bladder as its a one way track. Unless there is a problem with your urethra which i highly doubt at your age and no history of trauma or operation. Blueballs is an options and it could happen

No need to worry: There is no problem if sperm goes into your bladder (but that is usually associated with orgasm). "Blue balls" is a colloquial term for discomfort that can occur with being sexually excited without ejaculating but is not dangerous. Almost all 16 yo boys masturbate and it can be a healthy outlet. See http://boysunderattack.com/masturbation.html (it has a Christian perspective but offers good info)