"CHLAMYDIA is one of the most common; curable sexually transmitted infections (STD). This infection is spread by sexual contact with another person or from the mother to the child during birth. Chiamydia CANNOT be caused by casual contact (toilet seats, hot tubs, saunas, or swimming pools). Chlamydia infections are often overlooked because the symptoms are very mild or are totally absent. If symptoms are present they may include irregular vaginal bleeding, pain or burning with urination or sex, vaginal discharge, or lower abdominal pain and fever. Males may experience swelling of the testicles, have a discharge from the penis and/or pain with urination. A person can have the infection, not have symptoms, and transmit it to someone else without knowing it."

From:

http://chs.ky.gov/publichealth/std_infections_page_2.htm

Make sure that both you and your girlfriend are treated...she may have had it without realizing it since it is often asymptomatic...particularly in women. She should also be evaluated for pelvic inflammatory disease (PID) as chlamydia that is not treated in a timely manner is a primary cause of PID...which can result in chronic pelvic pain, menstrual problems, painful intercourse, infertility, ectopic pregnancy...and even death in more severe cases. Also avoid sexual contact between you (and certainly with anyone else!) until you have both recived follow-up testing and given a clean bill of health...you can pass it back and forth if you are not both successfully treated.