That's why she help him against Chimene Father.
Not only but also, the Infanta of Corneille's LE CID, had feelings for him.
La Infanta was Urraca, and lady of company was her younger sister Elvira.
The things goes wrongs, when Rodrigo support Sancho the older brother and the true heir of the crown against Urraca and her brothers.
Urraca's in her father will inherited a city and more than half of the monasteries, that means all the money in an institution called El Infantasgo, also the title of Abess-Queen. She concerved her priveliges if she remained unmarried.
Urraca protected Sancho's killers, and then she ruled Castille-Leon-Galice with her brother -some suggest as the movie incest- until she becomed old and enter in her nunnery.
FYC The Silent House best foreign film

Urraca definitely had a thing for Rodrigo and Chimene's behaviour throughout the entire situation after her father is killed well and truly made Urraca's blood boil. It's clear she felt Chimene was undeserving of a man so honourable and noble because her actions made it look like she didn't appreciate or recognise what kind of person she was passing up.
Urraca offered her colours because otherwise Rodrigo would've entered the duel without any, a terrible breach of protocol and also as a wake up call to Chimene about her coldness and unfair treatment towards him.
Oh and no, you're certainly not the only one who thought Genevive Page looked absolutely scorching hot as Urraca.