I have 40-50/hpf of wbc in my urinalysis and my left testicle is swelling. should i conclude that this is sti and tell my girlfriend now?

Absolutely not: I suspect you have epididymitis if the "testicle" is tender. I'm sorry your physician isn't better at explaining things -- get back on the phone and insist on an explanation for what's going on. I doubt this is a sexually transmitted disease, though a culture for gonorrhea and a chlamdia test may be in order.

Here are some...: The limited info suggests you might have suffered from UTI-related left epididymitis because STD-related urethritis tends to have much lower WBCs in mid-stream urine. Nonetheless, a combo of detailed Hx stressing features and sequence of symptoms over time + proper physical exam + timely tests including urine culture, US, etc will come to rescue of making a correct diagnosis. So, see Doc timely.