Why do I feel a burning sensation in my chest and a push when I say "Sayyid al-Istighfar" (the master of seeking forgiveness) or "La ilaha illa Allah" (there is no god but Allah) after waking up from sleep?

It is not possible to determine with certainty the cause of what is happening to you. However, dhikr (remembrance of Allah) upon waking up is religiously prescribed. It is better to adhere to the established adhkar (plural of dhikr) and supplications and not to be bound by what has not been established, such as Sayyid al-Istighfar (the master of seeking forgiveness) upon waking up at night, as it is among the morning and evening adhkar.

Among the adhkar reported upon waking up are: 1. Hadith: "Whoever awakes at night and says: 'There is no god but Allah, and Allah is the Greatest, and there is no might and no power except with Allah,' then says: 'O Allah, forgive me,' or supplicates, his supplication will be answered. And if he performs ablution and prays, his prayer will be accepted." Narrated by Al-Bukhari. 2. Hadith: "When one of you wakes up, let him say: 'All praise is due to Allah who restored my soul to me, and gave health to my body, and permitted me to remember Him.'" Narrated by Al-Tirmidhi and Al-Nasa'i. 3. Hadith of Al-Bara' bin 'Azib: "When he woke up from his sleep, he would say: 'All praise is due to Allah who gave us life after He had caused us to die, and to Him is the resurrection.'" Narrated by Muslim. 4. Hadith of Ibn 'Abbas: "When he woke up at night, he would wipe the sleep from his face with his hand, then look towards the sky and recite the last ten verses of Surat Al-Imran." Narrated by Muslim.

You should be diligent in observing these adhkar, in addition to the adhkar for sleeping and evening, as they are a protection from the evil of devils and others. - Ayat al-Kursi upon sleeping: "There will remain for you a guardian from Allah, and no devil will approach you until morning." - "Say: 'He is Allah, [who is] One,' and the two Mu'awwidhatayn (Surahs Al-Falaq and An-Nas) when you enter the evening and when you enter the morning, three times, they will suffice you against everything." - "Whoever says when he enters the evening: 'Allah is sufficient for me; there is no god but He. Upon Him I have relied, and He is the Lord of the Great Throne,' seven times, Allah will suffice him against what worries him of the affairs of this world and the Hereafter."