Fasting makes her lazy and causes her to lose weight; can she not fast?
Allah, may He be exalted, has granted a concession to the sick person allowing him not to fast in Ramadan. Allah says (interpretation of the meaning):
“whoever is ill or on a journey, the same number [of days which one did not observe Sawm (fasts) must be made up] from other days. Allaah intends for you ease, and He does not want to make things difficult for you”
The category of those who are ill includes the one who fears that he will become sick as a result of fasting, as has been discussed in the answer to question no. 12488.
Ibn Qudaamah (may Allah have mercy on him) said:
The kind of sickness that makes it permissible to break the fast is severe sickness that will be made worse by fasting or it is feared his recovery will be delayed. … The healthy person who is afraid of becoming sick if he fasts is like a sick person who fears that his sickness would become worse with regard to the concession allowing breaking of the fast, because the sick person is only permitted to break the fast for fear that his fasting would lead to his sickness becoming worse or lasting longer. The fear of the one who will become sick comes under the same heading. End quote.
Sickness is known either from the report of a trustworthy doctor or from the person’s own experience, if he fasts and finds that fasting harms him or causes great hardship for him.
You said that you do not suffer from any illnesses and that the doctors said that your weight loss is natural and not due to sickness.
In that case, if the hardship that you encounter because of fasting is fairly severe, then this is an excuse for you not to fast. But if you can put up with the hardship and get used to it, then it is obligatory for you to fast.
The Standing Committee for Issuing Fatwas was asked:
I had a daughter who was physically weak. The month of Ramadan has come and her mother did not let her fast the month of Ramadan for the past two years. The girl has died still owing two months of fasting. My question is: is there any sin on her mother because of that, because she was the cause of that? Does she have to make up the fasts on behalf of her daughter?
If this girl was not strong enough to fast because of her weakness, then she comes under the ruling of one who is sick, and her mother did not sin by preventing her from fasting the month of Ramadan. If the weakness continued and she was not able to fast until she died, then it is not obligatory to make up the fasts on her behalf.
But if the girl was able to fast, despite her weakness, with no undue hardship or difficulty, then her mother did sin by preventing her from fasting Ramadan, and it is prescribed to make up the fasts on her behalf. It is better if her mother does that because she was the cause (of her not fasting). End quote.
Fataawa al-Lajnah al-Daa’imah, 10/376
Our advice to you before you decide not to fast is to take measures that will help you to complete the fast, such as always eating sahoor at the end of the night, and not exhausting yourself with any hard work and the like during the day in Ramadan. Get enough sleep and rest, and consult a skilled doctor about your case; perhaps he will give you some medicine or tonic that will help you to fast.
And Allah knows best.