שמואל א א:ו פירוש: רש״י, Metzudat David ורד״ק

Her rival. Her husband's other wife, Penina.

And her rival would anger her: Peninah, her rival wife, would anger her with words.

Her rival (tzarah): Peninah, who was a tribulation (tzarah) to her. And likewise, all two wives [married] to the same man are called tribulations (tzarot), since they are generally each other's enemies. And likewise in the Mishnah (Yevamot 1), "exempt their tzarot."