bleeding caused by sickness does not invalidate your sawm, and you do not have to do anything even if the blood is a large amount, so long as it comes out without you doing anything to cause it.
The specification concerning the blood that invalidates the fast are as follows:
If the blood comes out as the result of a person’s actions or choice; this is subject to further discussion:
(i)If the blood comes out by means of cupping, this invalidates the fast, because the Prophet (peace and blessings of Allaah be upon him) said: “The cupper and the one to whom cupping is done both break their fast.”
(ii)If it comes out by a means other than cupping, such as venesection (opening a vein). If the amount is large and that has an effect on the person’s body, then this invalidates his fast (such as donating blood). If the amount is small and it does not harm the person, then it does not invalidate his fast, such as the small amount of blood that is taken for testing.
If the blood comes out through no deliberate action, such as an accident or nosebleed or an injury to any part of the body, his fast is still valid even if the amount of blood is large.
This is the summary of a fatwa issued by Shaykh Ibn ‘Uthaymeen.
But if the blood that comes out through no deliberate action is of such a large amount that he becomes weak and unable to fast, then it is permissible for him to break his fast and he should fast another day to make up for it.
