The secret message refers to incident of the Prophet (salallahu alayhi wassalam) drinking honey. This is mentioned in the tafsir of Ibn Kathir with several ahadeeth.
What happened was that the Messenger of Allah (salallahu alayhi wassalam) used to stay with Zainab bint Jahsh and drink honey at her house (A later narration mentions it could be Hafsah). Ayesha (radiallahu anha) and Hafsah (radiallahu anha) agreed that if the Prophet came to either of them, they would say to the Messenger of Allah (salallahu alayhi wassalam): 'I detect the smell of Maghafir (a nasty-smelling gum) on you; have you eaten Maghafir?' He came to one of them and she said that to him. He said: 'No, rather I drank honey at the house of Zainab bint Jahsh, but I will never do it again.'
Then the verse was revealed : "O Prophet! Why do you forbid (for yourself) that which Allah has allowed to you, seeking the approval of your wives?." referring to the incident that the Prophet (salallahu alayhi wassalam) forbade for himself drinking honey after one of his other wives asked regarding it. This verse appears a verse before the verse you mentioned in question. Usually, there is a coherence between verses when it refers to incidents like these.
Then Allah says referring to Aisha (radiallahu anha) and Hafsah (radiallahu anha) "If you two turn in repentance to Allah, (it will be better for you)". It is also mentioned in the narration of Umar that the verses were regarding Aisha and Hafsah (radiallahu anhuma).
"And (remember) when the Prophet disclosed a matter in confidence to one of his wives" refers to the Prophet (salallahu alayhi wassalam) saying: "No, rather I drank honey."
This has been recorded in several hadeeth in Sahih al-Bukhari, Sahih Muslim, Sunan Abi Dawood and Sunan An-Nasa'i source1 source2 source3 source4 source5 .