Demoted? Umh what?

Lately I've had a quite good MMR, given the fact that I gain 22-23 per win and lose 16-18, no matter the division. I sometimes skip promos and divisions, as well. I can also be at 0 LP, lose 5 games in a row after the demote prevention ends and still stay at 0 LP, and then still win 22 LP, so my MMR isn't really affected.

This is where the fun story begins: Here I am, in Silver 1, at 30-40 LP, can't remember exactly. I lose a SINGLE game and instantly get demoted to Silver 2, @60 LP. I start scratching my head, confused... OK, life goes on. I win a game, I win another, I lose one or two, win, lose etc. The first rule still applies: I lose 17 and get 22. I am now S2, 40 LP. I lose a single game, again.... and as you have already guessed, I am demoted to S4. YYYYYEEEEEP. 2 divisions for losing a game, even if I have a good MMR.

So, fellows, is this a bug or...? Hmmmmmmmm.