We have previously explained several times on our site that some kinds of cosmetic surgery are permissible, namely that which is done to treat a fault, remove a deformity and so on, and some kinds are haraam and are not allowed, namely that which is done solely for the purpose of beautification.
So long as the surgery that you are going to do is to remove a fault that resulted from losing weight, there is nothing wrong with that if it will not result in harm that is greater than the harm occurring right now.
The scholars of the Standing Committee for Issuing Fatwas were asked:
One of my friends got married by the help of Allah. He came to me and said: e came to me and said that his wife wants to have cosmetic surgery on her face and breasts, because her nose is large and wide, and she wants to reduce it by means of the easy methods that modern science has developed. Is there any doubt about this surgery or is there any sin involved? Please note that not doing it may lead to psychological distress because this fault appears so prominently on her face.
They replied: If the situation is as described, and there is the hope that the surgery will be successful and will not lead to a greater or equal harm, then it is permissible to do it in order to achieve the desired purpose, otherwise it is not permissible.
Fataawa al-Lajnah al-Daa’imah, 25/59-60
Shaykh Ibn Baaz (may Allah have mercy on him) was asked:
What is the Islamic ruling on having surgery to remove physical defects that may occur in people whether as a result of sickness or an accident, or that had been present since birth, such as removing extra digits and repairing the hand to make it look natural, or removing extra teeth and straightening the rest of the teeth so that the mouth will go back to normal, or correcting a harelip and making it look normal, or removing scars caused by burning, straightening a crooked nose or reducing its size in order to facilitate breathing, or completing a defective ear, or tightening drooping eyelids because they block one’s vision, or tightening sagging skin on the face so that the face will look more natural, or breast reduction for women in cases where large breasts are causing spinal problems because of their weight, or tummy tucks in the case of a sagging belly and weak abdominal muscles which lead to hernias, straightening the urethra in males in cases where urine is contaminating the clothing, removing deformed spots from the face, and removal of fat in obese individuals which may cause many diseases such as diabetes, high blood pressure and high cholesterol levels. Please note that these operations will not cause any deformity, in sha Allah.
There is nothing wrong with treating the problems mentioned with remedies that are prescribed in sharee‘ah or with permissible remedies from specialist doctors who think it most likely that the treatment will be successful, because of the general meaning of the shar‘i evidence which says that it is permissible to treat diseases and illnesses with acceptable or permissible remedies. End quote.
Majmoo‘ Fataawa Ibn Baaz, 9/419-420
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked:
What is the ruling on cosmetic surgery that is done by women?
If it is done to remove a fault, there is nothing wrong with it so long as it does not cause any harm, because ‘Arfajah (may Allah be pleased with him) wore a nose made of gold when his nose was cut off. One example is tightening or lifting drooping breasts. End quote.
Thamaraat al-Tadween, p. 133
And Allah knows best.