If you lose to people with higher MMR consistently and only win against people with lower MMR then your own MMR will likely start to tank. The problem is that the first part of that is far heavier then the second.
By that I mean if you play with someone, a duoQ or premade of some number, who are all lower MMR then you and you win half your games, your MMR will still likely tank cause you're playing at a lower one with your group pulling it down.
This happened to me a couple seasons ago, I was like Gold 5 and I had a friend in Bronze 3 or something, and back then there were no restrictions on tiers for duoQs. I carried him out of bronze and into silver, won like 75% of my games, and my MMR still tanked! I was forced to only play ranked with some plat-diamond friends of mine in order to get it back to normal, and by the end of the season I was able to climb on my own again without the system punishing me for playing with a friend in bronze.
So maybe that could have happened to you? Otherwise you've just lost to people better then you consistently and the system is trying to force you back down to the MMR of the people who you are beating.
