fuck boy problem :(

Okay so one of my so called best friends slept with me and we have always been flirty and what not. After he slept with me he totally forgot I existed, even though we were close friends. Anyways I've always sort of liked him even before he slept with me. About a week ago he messaged my best friend wanting to sleep with her! I'm kinda pissed off but he's not mine and I know I can't do anything about it. But really? My best friend? Do I have any right to be upset about this or am I overreacting?