Help, I win big, then lose big. . .

I'm getting tired of this repetitive crap, I get demoted to plat2, go on a big win streak, and then get to plat1 , go on a lose streak to get demoted again, rinse and repeat. Is there real any reason to this? Or is my matchmaking luck just trolling me? This has been going on for almost 2 weeks, my mmr is fine, it's around 55% win rate.

HELP