1ronKeys
Most games use some form of hidden MMR with matchmaking to try and place you in games with others around that same level.
If you're doing poorly that will be dropping, so eventually you will get into matches with others at that lower MMR, giving you easier matches for a bit. It sounds like that's not indicative of your actual skill level, so you're doing very well in those lower ranked matches, which means your MMR climbs, and you're back up to the higher difficulty lobbies again.
There's nothing that would give "free wins" just because you've made a purchase or used a new character specifically, this is pretty much always going to be tied to that MMR changing in the background. Essentially the game's thinking, "Oh you're doing great, time to bump you up to the next set of lobbies and give those people you've been destroying a break, you don't actually belong in this level."