There is NO way that gonorrhea can "develop on its own." I hate to tell you this, but she has been having sexual contact with someone or something (sex toy, etc) that is infected. It's far more likely that it's due to contact with someone else.
What else could it be? Far too many things to list on here. The only way to know for sure is to see a doctor for an STD screen. I would highly recommend getting yourself tested and ending that relationship. Should you have contracted it as well, treatment is very simple. Also, do not have sexual contact with her until she has been tested and/or treated.
im not sure, but it sounds like she cheated on you