It is your body telling you that you can not break a fast with an apple.

How long have you been fasting ?

Fibers in apple may irritate your stomach.

Try just some freshly squeezed juice next time. You can also juice things with your mouth, suck the juice and spit out the fibers. Fibers can be like a stone inside a stomach after a long fast.

Also, eating a whole apple at once can be too much, simply too big quantity for a first meal.

The best way to break a fast is just small amount of fresh juice or fresh veg. soup.

WS

I recently broke a 21 day fast with watermelon. I ate it every 2 hours for a week and then started juicing apples, spinach and carrots only the week after. 3rd week was salads with nothing more than lettuce and tomatoes with lemon juice and olive oil. By the 4th week I introduced homemade bone broth. I was instructed by physcian to go very slowly. Depending on the length of the fast, it could take another 30 days before you eat solid foods. Remember, your GI track is squeaky clean and practically brand new. So you have to introduce foods very slowly. Congrats on completing your fast!