Why is my second playthrough different and harder?

I was playing as a Thief. Everything was going fine, I had some pretty good items (even though I was only at the beginning at the game), and I had been getting pretty close to beating the second boss, Vordt of the Boreal Valley, that looks like a dog with titanium armor.

Then, I decided to make a new profile as a Pyromancer. I started seeing new paths and areas that I never saw before. So I thought, "Okay, no big deal. I just never noticed them before". Then enemies started seeming tougher, and some paths that got me to the second boss pretty quickly just vanished. Now there are some enemies that were never even there in the first place, and some enemies just up and vanished. I'm super confused, and if anyone knows if this has something to do with my vocation, or if the maps are randomly generated, please let me know.

For example, when you enter the High Wall of Lothric, there is a staircase leading down to a fire. In that area are two more staircases: one leading to a dead dragon and another room, the other leading to a dead end with a few enemies. The path leading to the dead end wasn't there when I first played, and I checked specifically for it when I did. There was also an area where a Mimic was sitting in the middle of the room. Playing the second profile it wasn't there. And the enemies are getting harder to beat, some enemies suddenly have more attacks, more health and can deal more damage.