Is it necessary to make up the day that I break my fast due to illness, along with expiation, or only by making it up? And is it permissible to break the fast if a stomach illness prevents eating and causes lethargy?

If fasting does not become difficult due to the illness, then it is obligatory to make up the day along with repentance, because breaking the fast is not permissible in this situation. Expiation (kaffarah) is not required except for breaking the fast through sexual intercourse, according to the preponderant opinion. It is not permissible to break the fast merely due to feeling tired. As for injections and blood tests, they do not invalidate the fast. As for IV fluids, if they are nourishing, they invalidate the fast; otherwise, they do not.