I'm not a Vet, so don't take this as gospel, but when I had a female turtle and her rear end got larger, I solved that by putting a male turtle in the tank with her.. I *THINK* it's a sign of the Turtle going in to heat....

This could also be very serious!Read what i have below. Turtle’s rear end is swollen, this is one of many possible indications of kidney failure (although it can also mean any of a number of other serious conditions.) If it is kidney failure, only palliative care is possible. Some other conditions can be treated. If you have several turtles, and one has kidney failure, it may be a sign that the animal has been feed too much protein. Change the diet of your other turtles. If your turtle is female, and you gently feel the area around her rear, and you feel lump, she may be graven and need to lay eggs. From another question here: Female Turtle is Restless or lays eggs in the water she doesn’t need to have been with a boy to need to lay eggs. If she hasn’t been, the eggs will not be fertile. However, she still needs a warm, root-free, deep( the depth needs to be about equal to the length of her shell, or slightly more) moist, sandy soil to lay the eggs in. If you fail to give your female turtle a suitable place to lay her eggs, she is likely to become eggbound, which will lead to her death if she is not treated by a herp vet. Most will give Oxytocin to stimulate the passing of eggs. The Oxyocin should be preceded by heavy doses of calcium for a few days and extra water. THE VET SHOULD NOT BE REACHING IN TO THE TURTLE FOR THE EGGS. If the egg has become heavily calcified and has to be removed by the vet, this will require surgery, and the turtle should be sedated. THIS IS SERIOUS AND DANGEROUS SURGURY.