I ate a chocolate then i saw that it's expiry date end from 15 days. is their any thing to worry about?

Not at all: Expiration dates are generally artificial; they often mean the time the item is best before but there is no harm in eating most foods after their expiration date unless they are clearly spoiled (such as moldy bread or sour milk). Expiration dates on drugs are usually meaningless and this often applies to foods too. See https://www.propublica.org/article/the-myth-of-drug-expiration-dates

No: There is nothing to worry about what you described. Have a healthy diet, drink plenty of water, maintain healthy weight, exercise at least 150 minutes per week. Wish you good health.