Instant Derank?

I won 7 games in a row and got promoted to gold 4 almost instantly once I entered gold, and I checked my mmr on op.gg and it said i had somethign like 50 over, I know it's not exact but its a good ballpark range for my in-game mmr. I then lost 4 games in a row then had a 50 percent wr out of 10 games and went from 70 something LP to 0 even though I had a win/loss ratio of 50 percent. I then lost a game at 0 LP and had an instant derank to gold 5, I have no idea how my mmr dropped so much since i played okay every game and had most damage most games and placed lots of wards and had a high KDA. I feel like the derank is so strange since i lost like 6 games at 0 LP in gold 5 before my huge winstreak. Just my thoughts on the wierd mmr system in league. Edit: im gold 4 again off of 3 games lol they really don't understand.