Ketoconazole is therapy for fungal (yeast) infections, not for trichomoniasis. Metronidazole, which you took is the appropriate therapy. Alternately there is a drug called tinidazole which is also highly effective for treatment of trich.
That your tests were negative is not suprising. As your doctor correctly told you, trich is difficult to diagnose in men. Treating you was the right thing to do.
Your GF needs to be treated with effective therapy other than ketoconazole before you have sex again. EWH
Thanks very much.
We can go back and get the right therapy organised