Good call man, just go out and have fun and don't worry about her. Plenty of girls out there, if you do want to keep your ex as a friend with benefits then simply treat her like one. If she replies angry and frustrated, clearly she's frustrated at you for not acting like you care about her. That's good because you got the upper hand. Just casually email her back, not even responding to her frustration like icup said. Let her flirt with you, and when she flirts, flirt back. Tease her sexually and go on about your day doing whatever you usually do. Let her be sexually frustrated, keep on teasing her and flirting with her. Keep your composure, if you keep this up, she'll be visiting you

Women love a challenge, challenge her.

If you tug a ball of yarn in front of a cat, she will chase it. Good luck bro
- Nelson