ok here's a question

ok here's a question, if your out with friends just hanging out and your single, why the hell would one of your female friends get pissed and storm into the bathroom after you start flirting with another female? this friend has never shown any intrest in me other then being friends but when i started talking to this female at the bar, she got all pissy i just don't get it. am i going nuts? or whats going on?