Well when you go on a stomping spree that'll happen, since the game is remarkably good at pegging smurfs as smurfs and matching them with either other smurfs or relatively high rank players. You most likely do not actually have a low mmr despite being a low level.
If it's overshot your true mmr (pretty easy to do since it start compensating pretty hard early on when you're on a smurf), then you'll lose a bunch of games and fall to a lower mmr. Keep at it and eventually it'll overshoot/undershoot by less and less until it eventually settles down to around your mains mmr, although it'll probably take 100ish games.