I am really confused with this whole MMR thing

So I just got promoted to Bronze 4, and have lost 2 games at 0 lp, not demoted yet. And my MMR is down to 898? It was not like I was feeding in those losses, and had just came off of a 6 game winning streak. This is weird because I was 1/5/14 as support in the 1st game, which isnt great, but I wasnt feeding. Then In my second game I was 12/3/27 in a 60 minute game. And now my MMR is at bronze 5 level? Last time it was at bronze 4 level, on my 1st promotion game. I am pretty scared I will be demoted next game due to an MMR that I don't think is right. I don't know, this is according to my op.gg, and I don't really understand the mmr thing.