What is up with my MMR?

Ok, so I started playing this game again after some years. I play the placements and I do terribly, so I end up Bronze V. After a while I have a win streak and I end up Bronze IV 54 lp. When I won the first game in Bronze V, I got 21 lp. After some games, I started getting 19 lp. Now I'm getting 18 lp. I didn't lose ANY games at all, and yet I kept getting less and less lp the more I was winning. And now I lose 1 game and I lose 20 lp. What is happening.