Demoted for no reason

I was gold 3 about 30 lp or so, played some games with a friend who was plat 3. We won every game we played together. Got to gold 2 promos, riot gave me a free win so I only had to win one more game to be gold 2. I won one more, skipped gold 2 entirely and went to gold 1 18 lp. Seemed reasonable to me, whole time i was playing with and against people in plat 4 and plat 3, me being the only gold in the games and I won lane every game, carrying with my duo. I log on today, about a week after getting to gold 1, and I'm now at gold 2 0 lp? Why was I demoted. I know its only preseason but that had me pretty tight.