What is the cause of the tightness in my chest and the sadness that afflicts me after Fajr prayer until 10 AM, from the second day of Eid al-Fitr until the sixth or seventh day of every year?

The cure in the Holy Qur’an is through the remembrance of Allah Almighty: (Those who have believed and whose hearts are assured by the remembrance of Allah. Unquestionably, by the remembrance of Allah hearts are assured) and (And We send down of the Qur’an that which is healing and mercy for the believers). One must strive to live with the Qur’an and ponder its meanings, for Satan seeks to distract one from this. Do not preoccupy yourself with thinking about the reason for what is happening, but rather turn to Allah with sincere and certain supplication, and present yourself to a doctor or a ruqya practitioner whose religion and piety you trust.