MMR and its mechanics.

Can someone please explain to me a way to balance this MMR situation:

I am plat 5, I lost 4 games in a row once due to feeders and 2 afks. My mmr lowered to that of 3 divisions lower than me NOT MY FAULT. Trust me I don't feed and often times I can carry games single handily. But in all I MAJORITY (overall) win more than i lose. How is my mmr so bad I play with gold 5's while being plat 5?