In the fourth year of King Jehoiakim son of Josiah of Judah, this word came to Jeremiah from the Lord: Take a scroll and write on it all the words that I have spoken to you against Israel and Judah and all the nations, from the day I spoke to you, from the days of Josiah until today. It may be that when the house of Judah hears of all the disasters that I intend to do to them, all of them may turn from their evil ways, so that I may forgive their iniquity and their sin. Then Jeremiah called Baruch son of Neriah, and Baruch wrote on a scroll at Jeremiah's dictation all the words of the Lord that he had spoken to him.The Gemara (Mo'ed Katan 26a) states that this scroll was the short book of Eicha (Lamentations), and Rashi (Jer. 36:23) follows this explanation. R. Yosef Kara (Jer. 36:23) finds this difficult because the command was to write "all the words that I have spoken to you against Israel and Judah and all the nations, from the day I spoke to you, from the days of Josiah until today." Eicha simply does not have all this information. Rather, it must be referring to a scroll containing all of Jeremiah's prophecies up to that day.
Abarbanel (Jer. 36:2-3) responds that Eicha does contain grieving over the status of Israel and Judah, in terms of their destruction and exile, and also mentions God's revenge on the nations that exiled them (e.g. Lam. 1:22 "Let all their evil doing come before you"; 4:21 "Rejoice and be glad, O daughter Edom..."). [Note that the Metzudas David follows Abarbanel's approach without citing him by name, as is his way.] Interestingly, the note in the Net Bible says as follows: "The intent is hardly that of giving a verbatim report of everything that the Lord had told him to say or of everything that he had actually said. What the scroll undoubtedly contained was a synopsis of Jeremiah's messages as constructed from his memory." This could more plausibly be applied to the Gemara's approach than R. Yosef Kara's.
Abarbanel continues that Jeremiah generally wrote down his own prophecies, that had the status of nevu'ah (prophecy) which compelled him to express them. However, the lamentations included in Eicha were said under divine inspiration (ru'ach ha-kodesh), a level lower than prophecy, which is why specifically those were written down by a scribe and is why they were not included in the part of the Bible called Nevi'im but, rather, Kesuvim. On this issue, see R. Shnayer Z. Leiman, The Canonization of Hebrew Scripture, pp. 169-170 n. 294.