I had the same experience with my alt account. In the beginning I had a 90% win rate with an aggressive DH build I threw together just to try and climb quickly.
After 50 games or so I started getting run over by tier 1 decks and was confused as to how a beginner could craft such decks. The reality is you're building an mmr right from the beginning. With a great win rate, matchmaking starts to put you against opponents of the same mmr regardless of rank. I didn't realize it at the time, but I was playing against diamond ranks. Once you get through the apprentice ranks you're awarded a star bonus, just as you are at the start of each month. I was given a 10 star bonus which is the moment I realized why I had been facing such stiff competition.
I don't think my winrate was quite that high, but this is an interesting/troubling proposal. Are you certain that the people you were queing into when you were climbing through the apprentice ranks were in diamond? The people that accepted my friend requests were all also in the apprentice ranks, but it's possible that some of the people that didn't accept were diamond.