MMR is Wack

I normally don't do ranked, I got placed Silver 3 and didn't have time to grind. However once my friend got to silver 1 I duoed with him and carried him into gold. By this time i was silver 2 with a 80% win rate and figured... ya I would like gold as well since I did all the hard work. I kept duoing with my friend. Now I am in my gold promos and my friend is gold 4, but because I have maintained my 80% win rate and now apparently have a gold 3 mmr while in silver1. I am now facing full teams of S7 plats IN MY GOLD PROMOS. Rito Please WTF????