Diabetic dog with bloody urine on weekend, should I see an ER vet?

UTI's can cause your dog pain, that's for sure. If you've ever had one yourself you might know that the burning sensation and pain in the lower abdomen are quite uncomfortable. The pain isn't terrible, for sure, but there's no doubt that Bella isn't feeling well. In addition, in a diabetic dog, UTI's can indicate that she is poorly regulated (i.e. there is routinely glucose in her urine) and having one signals that it's a good idea to check blood glucose as well, to determine if an adjustment might be considered in insulin dosage. Checking a quick urine dipstick can also ease your mind that she is not tipping over into diabetic ketoacidosis as well, which is a very serious condition.

It's also worth noting that even though the same antibiotic is prescribed each time, when UTI's occur frequently the bacteria that cause them can become stronger, and more resistant to the normally-used antibiotics - this is why urine culture is so valuable in these cases, in order to make sure that the appropriate antibiotic is used. I would recommend that you get her in today, since it's another 36 hours since your regular vet opens, and that's a long time for her to be uncomfortable.