Since there are lot of questions about how matchmaking and MMR works, why you lose matches as soon as you won some earlier, why you're getting matched with/against worse/better players, etc, I want to copy-paste Riot Lyte's answer to a question about matchmaking, and give some explanation by myself.
"[...]_When you win matches, your MMR goes up, and you face tougher and tougher opponents (which means eventually, when you reach your true MMR potential, you'll start losing about half your matches and winning half your matches). When you lose matches, your MMR goes down, and you face weaker opponents until you start winning your matches. So if a player is smurfing, they are playing matches against much lower skilled players and will win a lot of matches in a row and be on a win streak. Their MMR will increase dramatically, and they'll face tougher and tougher opponents until they are at their true MMR level, and will start losing a lot more games until their win percentage stabilizes around 50% (because their MMR gains have stabilized, not because the system forced a 50% win rate on them)[...]_.
So when you win some matches, eventually you'll lose some because you start playing against even better and better opponents, and your actual MMR can be higher than your true MMR. Once you go on a win streak, you'll lost some matches, because your MMR skyrocketed up, and that's inevitable when a win streak happens. The reason why you're on a lose streak (tilt) is because previously you had a pretty big win streak, and your MMR became higher than your true MMR is, so it takes time (and matches) to normalize your MMR.
And very important: Normal games are different than Ranked games in terms of MMR. When you play normal games, you have absolutely no reason to watch the enemy's tier and division, because that shows his/her ranked stats. This is why sometimes there are bronze and diamond players in the same match, because their normal MMR is similar, it's just the ranked MMR that's different. If you constantly play against Diamond tier players as a Bronze/Silver player, then there's a very big chance that your actual normal MMR is somewhere between Gold-Plat, so you can be extremely happy about it, since you can learn a lot from better and more experienced players, even if it's "just" a normal game, where people usually try new stuffs and practice for ranked.
So cheer up if you're on a lose streak, it's not tilt or something, it's just usually playing against better players thanks to an earlier huge win streak. Of course bad games can happen, but if you're pretty sure that you did your best and noone on your team trolled you, you have no reason to be sad. If you proceed to lose your games, eventually your MMR goes down, and you'll play against people with the same level as you. And don't forget that playing against better players have high impact in your future (ranked) games, as you can only learn from better players.
Have a good day, have a good game,