Flirting with Girlfriend's friend

I'm in a relationship with my my gf for two years. Our relationship was good, but it had become a bit boring. That’s when I started noticing her best friend, say S

S was always around.She joked with me a lot, looked at me in a special way, and sometimes sat close. Slowly, I started liking the attention. And s was giving me more of it. We started chatting at night. At first, it was normal talk“Are you awake?” or “What’s up?” But slowly, it became more flirty. We teased each other, said bold things, and kept chatting late into the night.

During the day, when my gf was around, we acted like nothing was going on. laughed, went out together, and looked like a normal group of friends. Sometimes I would send her a message with a naughty smile emoji while my gf was sitting right next to her. She would read it and feel excited like we were doing something wrong, but thrilling.

We never talked about leaving our partners. We are just enjoying the feeling, the secret moments, and the excitement of doing something no one else know about.

Anyone wanna chat about it can msg