Hi and welcome to the site. My name is Dr. C. and I'll be providing your medical information today.
Yes it is possible for an untreated UTI to cause lower back pain and abdominal pain. Drinking a lot of water and taking cranberry extract may flush a bacterial infection our of the urinary tract.
But it's also possible that there is a bladder infection that requires antibiotics or another cause of the pain. For example, it there is a possibility of pregnancy, a home urine pregnancy test should be taken. Some women will have complcations from pregnancy even without seeming to skip any periods.
If the pregnancy test is positive and the above measures don't take care of, beeing evaluated for possible ectopic pregnancy would be important.
I hope it's a simple UTI that responds to water and cranberry extract.
Please REPLY if I can clarify anything further. I want to be sure you have the most helpful information possible.