Take your dog to the vet IMMEDIATELY. Your dog may have a bladder infection, which is swelling and causing her to sway while she walks. The fact that her tail is between her legs and she is whining is a clear indication that she is in pain. Many dogs tend to sway and favor one side of their body when they have infections in the rear of their body. Though unlikely, your dog may even have a gastro-intestinal virus. If her breath is excessively bad, or worse than normal, then she may have a problem with her digestive tract, which ironically CAN cause bladder control problems. You may also notice an abnormal, overpowering odor coming from the dog's rear, which is similar to the odor of the dog's breath. Note that the odor will be noticeable from a distance. If this is the case, then again, I advise you to waste no time in taking the dog to the Vet. Age is usually not a factor in disease...just because your dog is 3 years old doesn't mean she isn't susceptible to any kind of life-threatening illness. Also check to see if there are any venomous animals in your region...she may very well have been bitten by an insect or animal. Since I'm not veterinarian, I am only giving you the scenarios that I have experienced with my own dogs and pets belonging to family and friends. It sounds like she has a severe bldder infection. I don't want to come off as being rude when I say it, so don't take it that way, but the only advice I can truly give is this: Don't waste time on this site asking questions, and take the dog to a professional immediately. There are helpful people here, but there aren't any professionals on the site that can accurately diagnose the problem, and by delaying you are only prolonging the suffering of your dog. She is clearly in alot of pain and needs to be examined, because from what you are describing she may have a very serious illness. Although I hope my information was helpful, it is far from a professional diagnosis, its just personal experience. The dog needs professional examination as soon as possible. Good luck and hopefully everything will be ok.

I would say that you need to have her check by a urologist.