gynecological infections very commonly have similar symptoms. these include STDs (gonorrhea, chlamydia, herpes, etc) and non-STDs (yeast infections, bacterial vaginitis, etc). in either case, the problem is treatable. if your girlfriend's doctor didn't give her a pelvic exam, she should definitely go back and get one.
interstitial cystitis is also possible...it has a scary name, but it's not something to be overly worried about. bluemirage described it pretty well. it's actually fairly common, but it's cause is unknown. it's a pretty bothersome disease because of the symptoms, but it rarely causes problems other than the one's she's already been experiencing. it's usually recommended that the patient avoid consuming caffeine, alcohol, tomatoes, vinegar, citrus fruits and spicy foods because these are bladder irritants. if this doesn't work, there are some medications that her doctor can prescribe.
someone mentioned TB, but unless your girlfriend has severe problems with her immune system (is undergoing chemotherapy, has AIDS), it is EXTREMELY unlikely that she would have TB that has spread to her kidneys. she would also be sick as hell otherwise.
hope this helps.