No worries, Xant. I'm just a bit higher with my mmr than 3900.
After those 10 matches it also seems only to be about wins to change your mmr. Winning a game may give you +25, losing the next - 26, winning the next +24, and so on.
Guess more important was your normal mmr before as Thovex said...
I won 8 of 10 of the first ten matches, I'm mostly playing semi-carries and supports, position 1 rather rarely.