There are so many things that could be causing this and I'll give you the 'short' run down so you can make a compassionate and informed decision on what to do next.
First - probably the cause I see more of than others - is hepatic lipidosis or fatty liver disease.
Fatty liver disease is something that is often seen in a bird on a seed only or predominantly seed diet. No matter how much the manufacturer insists they are fortified and healthy, they are misleading all of us.
Skeletal problems, deficiencies and even toxicities can cause various symptoms too, even though they seem very unrelated. Anything from loss of balance in some birds, as well as the more common symptoms such as breathing difficulties/breathing with beak open, restlessness, nervousness, acting afraid of usual things, startling easily and so on.
Zinc and other toxic metals or substances can be ingested slowly over time when toys, clasps, chains, links or even cages are chewed on or played with. Other poisonings occur when the bird actually swallows a toy, link or piece of one. Watch out for bell clappers for instance.
Take a look here under critical conditions to reassure yourself that urgent veterinary intervention may be necessary (these symptoms apply to all birds, not just ‘tiels)
Remarkably, birds don’t need much, if any vitamin C. It is a water soluble vitamin which means it passes out of the body after the body takes what it needs and C is available in a wide variety of both fresh and processed foods given to birds.
Vitamin A is frequently found to be deficient in birds. This is a fat soluble vitamin which means it gets stored in the fat cells of the body, so it’s possible to overdose on it. With our companion birds though, too little is the situation most often encountered.
The symptoms a bird will show when deficient are increased allergic reactions, respiratory/sinus infections, reproductive problems, skin and feather disorders, even cysts and tumors, as well as various intestinal complications. These issues may also come with the symptom you're seeing in her leg.
Vitamin A is most ideally received in its natural form which is Beta Carotene (this cannot be overdosed on) from natural foods like sweet potatoes, yams, carrots, squash and other dark colored vegetables. If your bird doesn’t care for fresh vegetables, a ½ teaspoon of natural baby food (human baby food) of any of these vegetables. Again, it must be all natural and nothing but the vegetable with water sufficient for processing.
Beta Carotene converts to vitamin A 'as needed' in the body and what's not needed is pooped out harmlessly. So remember, never give a vitamin A supplement; always increase Beta Carotene rich foods.
A feeding option is to offer ½ spoon of all natural, organic baby food (squash, yams, sweet potatoes, mixed vegetables) which many birds take readily; also try some pabulum or baby rice cereal and a few licks of natural (no artificial anything) yogurt.
Feel free to mix these or offer them one at a time.
If you notice improvement after feeding these foods (within hours typically) make sure to note this to your vet. Yes, no matter what a vet will be necessary since these birds are excellent 'maskers' and can fake recovery expertly. By the time you see symptoms again, and you will, the underlying issue will be far advanced and the treatment will be complicated and costly.
Let's hope she has the most minor of deficiencies, easily fixed and another decade of companionship to spend with you, at least!