Well, if no urinalysis was done on you, I'd request that they check one to see if you have a UTI (urinary tract infection) since it can present the way you describe. Secondly, I'd wonder also about a possible STD (sexually transmitted disease), are you sexually active and if so, there is a chance it could be that. As I said in the first response, constipation as well as a gastroenteritis could be an issue. If you've had no loose or diarrheal stools, you could try a stool softener and see if the pain is relieved with that. Otherwise, a CT scan or a gynecological exam may be needed. If that doesn't show anything, I'd be about as clueless as you are now!
Are you having any urinary issues at present, like frequency, burning, or itching? Have you done any heavy lifting lately? Are you constipated? Are you premenstrual? All of these things could present with symptoms of back and lower abdominal pain.