She seems to have been a ruler in her own right, as well as a liege of Arthur’s.
When the King of Northumberland besieged her, she sent a messenger to Arthur, asking for a champion. The newly dubbed Lancelot craved to act for her. He made a side trip to rescue a damsel from a big and apparently cruel knight; this, however, turned out to be a test arranged by the Lady of Nohaut – the big knight had wanted to champion and wed her, and she had consented on condition he defeat the champion Arthur sent. (He did not.)
When Lancelot arrived at Nohaut, the Lady put him to bed for fifteen days to recover from his wounds. Meanwhile, hearing no news of Lancelot at court, Kay requested the errand and was sent to Nohaut in his turn. After a quarrel between Kay and Lancelot, the Lady settled the question of which was to act for her by requesting the King of Northumberland to allow her two champions.
They fought two knights of Northumberland. Lancelot, after vanquishing his opponent, offered to help Kay; Kay refused the offer and eventually succeeded in mastering his own adversary. Kay returned to court with the Lady’s thanks, while Lancelot lingered a while at Nohaut.
I strongly suspect, because of the name similarity, that the ladies of Nohaut and Noauz should be identified with each other.