Why should Yirmeyah tell them this?
Radak: When they hear that you cry and are pained over your Nevu'ah of the victims of the sword and hunger, perhaps their hearts will be broken and they will be humbled.
Malbim: He should tell them that Hashem joins in their pain. It is as if His eyes shed tears.
Why does it say "Besulas Bas Ami"?
Radak: Until she is exiled, she is like a Besulah, that no man was intimate with or ruled over her. The Goyim did not rule over Yisrael to exile them. Now she will be submissive to them in her exile, like a Besulah after she had Bi'ah.
What is "Nachlah"?
Rashi: It is an expression of Choli (illness).
Radak: It is a great blow, a harsh illness.
Malbim: This is the internal malady that accompanies Shever, which is external.
