If I have diarrhea and body is weak and feeling feverish, can I make up the fasts later? Also, do I just not intend to fast the day or do I intend to fast the day and break the fast later on in the day due to illness?

If you are sick to the point u can not fast then it is allowed for you not to fast and make it up later on. As for niyyah, it should always be decisive so if u can then u intend to fast full stop. But if u r pretty sick u can break your fast.