It's your hidden MMR fluctuating. As you may be aware, you do not face opponents that have similar rankings as you, but rather opponents that have similar hidden MMRs. This means that when you win a lot, your hidden MMR adjusts for you being well above your current ranking. So much so, perhaps, that you then go on a losing spree from it placing you into weird situations which then, of course, shifts you back dowards to your original MMR (+ some, likely).
It happens! Eventually you will likely go on a winning spree, not go on a losing spree, and you'll end up where you are meant to be.