Should i tell my best friend i slept wih this girl?

Not too long ago i slept with a friend of mine. We were both really drunk and the was great. However she made me promise i wouldnt tell my best friend because she would be mad at both of us. Tonight i saw the girl i hooked up with and she told me that she told her bff that she slept with someone she shouldnt have. as in me. she said she had no regrets but it was a little strange because we're friends. i agree. Should i tell my best friend i slept with this girl? It could cause a huge fight but its killing me not to tell her. i tell her everything.