Diamond Player in gold games?

For my past about 15 or so games ive been playing with around gold level players, and early on it was even against low bronze players and some mid silver players, i dont really know whats going on the only thing i can think of is because i like to play with my friends and they're around the silver-gold elo but even when im just by myself in normals i still get placed with players at about gold MMR but im diamond, i just kinda want an answer to this other than to just never play with my friends ever again because while i usually just stomp every game i also want to gouge my eyes out with the plays i continue to see.