She may be anaemic and this may have to be ascertained by blood Hb test. Inspite of good qty of food she needs to get green leafy vegetables, salads and juices and milk in order to maintain balance.
The reason could well be mal Digestion too not allowing nutrients absorb and assimilate.
Sore throat, if not treated, could become marked infective upper
respiratory tract. Mild affect of weakness can be felt due to this too temporarily.
Best way is to motivate her to do saline gargles with warm water and mix a pinch of turmeric to this water. Do thrice a day for a week or so.
If you can get holy basil leaves, boil these with little pieces of ginger and say her to drink two times at least. Avoid drinking
cold water, ice creams etc.
If you can make her to do deep breathing for about 5 to 10 minutes daily in fresh air, she will improve fast. In fact you have to do it so that she imitates.
If you are in India, give her 1/2 tsp of Tarunikusumakar chooran at night every alternative day to keep gut clean. Its not palatable so you can mix honey to it and lick..then drink little warm water. Out of India, you have to arrange it.
These are all safe and harmless.
Hope it helps. For any further query please feel free to contact me through direct question.