How Are Demotions decided?

I am really mad at the demotions system, I am just at a loss for the inconsistency. I just got promoted last night and i was like cool maybe I wont get demoted again. I play one game win(go up to 19LP) I am pumped I lose(down to 0LP) a game by a small margin still feel good about the day so I play one more its such a close game having tons of fun and lose demoted.... I really feel when you just get promoted there should be some kind of safety net because losing 1 game at zero is bull. But then the last time I got demoted I lost 3 in a row at zero and the 4 game demoted me!? what the heck is with the demotions sometimes I feel like its being really nice to me others its like screw you loki. I just really wish there was some consistency. speaking of which were did the warning go when you reach 0 I never get that now.