Torment only works if it's the last move used in a turn. I'm terrible at explaining so perhaps a step-by-step would help:
Turn 1
- Kirlia used Confusion!
- Shroomish used Giga Drain!
Turn 2
- Kirlia used Torment!
- Shroomish used Giga Drain!
- Kirlia fainted!
Torment does not count the last move used before Torment, which is why Shroomish was able to use Giga Drain twice. Should it have tried to use Giga Drain on turn 3 it would have failed.
TL;DR Torment only affects the move used immediately after it, not before it.
Here's Bulbapedia's answer:
>Torment prevents the target from using the same move twice in a row, starting from the turn following Torment's use. If Torment is used first in a turn, the move used by the opponent in the same turn is affected on the following turn. If Torment is used after any other moves are made in the same turn, it will be applied to the opponent's move made in the following turn.
Source: Experience and Bulbapedia
