If you have fasted before you do not need to worry. You can do some testing on your own. The only time you need to see a doctor is if you were vomiting for more than three days, chest pains with heart pounding, blood in urine or stool. Of course you can use docotor’s help in case of weaning off of drugs, extreme weakness, etc. Other than that, your body knows what to do when food is not coming in. Just rest, stay warm and drink warm water per thirst.
Also all fasts are different. I ha e stopped fasts when it just did not feel right. Listen to your body.