Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I am sorry to hear that Coco isn't wagging her tail, is holding it down, and is showing signs of pain when you attempt to touch it near the base.
I suspect that your girl may be suffering from tail head myositis. This occurs from overuse of the tail, usually after swimming, or playing and socializing more then usual.
It is a painful inflammation of the muscles at the tops of the tail which make the whole tail sore, and difficult/painful to raise. Sometimes, especially in short haired dogs we can even see swelling at the tail base.
Given some time, rest and anti-inflammatories she will feel much better and her tail should regain its normal happy wag.
If she is eating and drinking normally and has no history of bleeding tendencies or a sensitive stomach you can give her buffered aspirin at home for a couple days.
The only over the counter anti-inflammatory that can be used in dogs is buffered, enteric coated aspirin (like ascriptin). Aspirin does cause stomach and intestinal irritation and ulceration as well as clotting problems so should not be given for more than 2 to 3 days consecutively and should always be given with a meal. If you choose to use it watch for lack of appetite, vomiting, blood in the stools or dark tarry stools and stop immediately if you see those. Do not use aspirin if your dog has liver or kidney disease or a history of a sensitive stomach or clotting problems.
The dose for aspirin is 5mg to 10mg per pound of body weight orally every 12 hours (about one half of a 325mg aspirin for a 25 to 35 pound dog every 12 hours). Always give with a meal. Do not use for more than 2 or 3 days.
Other causes of a lowered, painful tail are usually traumatic (tail pull injury, hit by a car in the pelvis), disc disease, or a mass placing pressure in the spinal nerves that innervate her tail. But with no history of trauma and an abrupt change like she has tail head myositis is most likely.
Another possible cause of pain near the tail is full, impacted or infected anal glands. These scent glands are found just inside the rectum and when they are abnormal and uncomfortable the inflammation can spread up to the tailhead area. If she seems to be scooting her bottom on the ground or licking her perianal area it is worth having her anal glands checked by her veterinarian tomorrow.
Please let me know if you have any further questions.