Cystitis, also known as bladder inflammation, can lead to straining when urinating. It can eventually even lead to more severe and emergency type of situations like the formation of stones in the bladder or the formation of a urethral plug, which is a life-threatening condition that causes the dog to become "blocked" (i.e., unable to urinate).
Is your dog peeing for the umpteenth time today? Dogs with urinary tract disease often urinate an abnormal amount of times each day because little to no urine is being expelled each time. This is obviously very frustrating and also dangerous because when a dog is blocked they are unable to rid themselves of bodily toxic waste products through their urine.
The pain associated with urinary tract disease may be so severe that some dogs will lick their penile or vaginal area (or at times the abdominal area) as a way to try and self-soothe. Dogs with urinary tract disease may also be more irritable than usual.
Dogs with urinary tract disease will often have urine which is blood-tinged or discolored. Females are often at greater risk for urinary tract infections that lead to blood in the urine than are males.
Urinating indoors is not always a medical issue, but you should be concerned of it, especially when combined with any of the other aforementioned symptoms.
If your dog is exhibiting symptoms of urinary tract disease or you suspect something is wrong, schedule an appointment with your veterinarian immediately, especially if your dog has stopped urinating altogether (possibly due to being blocked). He or she will evaluate your dog and collect urine samples for testing. In some cases, blood testing may be required. X-rays and abdominal ultrasounds are also often necessary in order to diagnose the cause of urinary tract disease.