Just wondering about MMR

Okay so yesterday I finally made it to my platinum promos after I won 18 out of 20 games in a row, I was so happy. But then I went up against full plat 3 teams and lost my promos twice in a row and not just lost them, but lost them hard. I kind of tilted and had some more bad games, but, earlier today I had a game where I was still matched with a full Plat team, because my MMR is that high it seems? I even had a Diamond 5 (76LP) cait in my team who was really bad. Again we lost the game. 2 games later I check everyone's elo and guess what. Enemy team full silver 1 with 1 gold 4. Needless to say I destroyed that game completely, but how for the love of whichever deity you believe in, do you go from playing against full plats to playing against full silver in 3 games?

Also I had a game today with MMR 1514, I lost the game, next game I get a team MMR of 1300. Now I don't know but losting 214 MMR in one game?