Hi there -
1 - Not necessarily. You could get this from normal mouth bacteria entering your urethra from oral sex. If she had a cold or a virus, or strep, and gave you oral, you can get that in your urethra.
2 - Also possible. There are lots of bacteria - both naturally occurring and sexually transmitted - that cause NGU, and its possible you just got something from her that irritated your urethra.
3 - No. NGU alone is not even close to a reason to suspect infidelity.
Other causes of NGU - prostate infection or inflammation, urinary tract infections, strictures of the urethra, etc.
thanks that is helpful. i think i'll calm down again about it.