Doctor Help Nonespecific Urethritis

Hi doc, thanks for the job you do answering our questions.

Some time ago I had some type of infection in my penis, I had yellowish discharge and while urination it burnt.The results the doctor gave me said it was an nonspecific urethritis infection.

If have never had any other sexual relation, only with my girlfriend and she says she has not either (we have been together for 7 years).My girlfriend was tested negative and I positive.

1.- how could I have caught this infection?

2.- could it have been nonspecific urethritis instead of chlamydia or gonorrhea?Or another type of bacteria that caused the disease?

3.- does this mean it could be a disease that is not sexually transmitted?

4.- are all nonspecific urethritis sexually transmitted? I did not transmit it to my girlfriend

5.- what other ways of catching nonspecific urethritis are there?

The night before the first syntoms appeared I went out with my friends all night long, I didn´tthave sex or anything similar, no contact with anyone, except shaking hands, drinking out from their glasses...

6.- could my disease be caused by urinating or masturbating with filthy hands?

7.- why wasn´t she infected if I had a positive result and we´d had sex all the days before?

8.- if it were to be a sex transmitted disease, if I am sure I did not introduce the disease in the relation by sexual means, because I only had sex with her, could this mean that she had sex with some else during these 7 years?

For example, if she had a sporadic sexual contact, with another person that was infected, and we had sex all the time after her episode.
When would I have noticed that I was infected?
Would the infection be inmediate (1 or 3 weeks), or could I have noticed it 6 months after her contact with another person?

The facts are that I was infected and she was not. I know I did not have external sexual relations with other persons. And her analysis were negative. How come the disease reached me?
If she had another sexual contact outside our couple with someone else 6 months before. How come she did not pass me the disease till 6 months after her external contact?

Thanks a lot.