What is the meaning of "Yashomu"?
Rashi: They will be astounded.
Radak: They will have Shemamon (depression).
What is the meaning of "Al Ekev Bashtam"?
Rashi: They will be shamed in the full Midah that they meted out and in the way they came against me. "Al Ekev" is like "v'Ikevosecha Lo Noda'u" (77:20), "Tze'i Lach b'Ikvei ha'Tzon" (Shir ha'Shirim 1:8).
Radak: It is payment for the shame that they shamed me at the time of my affliction. Ekev is reward, like "Ekev Tishme'un" (Devarim 7:12).
Why do they say "He'ach"?
Rashi: When Tzarah comes to me, they boast of my downfall, and say so due to me.
Radak: They say so amidst Simchah. We find "He'ach He'ach Ra'asah Einenu" (35:21).
Malbim: They falsely slander me, and say "He'ach He'ach Ra'asah Einenu" (35:21).