My nine month old puppy has started urinating indoors again over the last three days even though the door was open and she could get outside easily. I suspect she may have a slight urinary tract infection but wondered if it could be anything else. She has been house trained for over five months.
If she is urinating more frequent than normal then there it is probably an infection as you suspect , especially since she has been house trained for such a long time. Symptoms of an infection are an increase in water consumption, frequent urination and possible blood in the urine. Many dogs that have an infection show only one or two of the symptoms an some show none at all. Since this breed is prone to so little ailments it is more likely an infection. I would catch a urine sample from the first in the morning if possible. Since she is small using a ladle may work out easier for you. If you can not obtain a sample then your vet can probably get one so don't fret over it. Here is more on urinary tract infection as well as your breed. Tract infection http://www.petplace.com/dogs/acute-cystitis-in-dogs/page1.aspx
Scroll down to prone illnesses, you'll see they are pretty hardy.