When Amy cut her foot open on the broken glass (what was it, a vase?) was it because of Doug? Did he set that up to make her more vulnerable and give him another "in"? It seemed to work in his favor, so I tend to think he did but I didn't see him do it. Maybe we're supposed to surmise that he did?
Afterward, when Amy met Doug, she said it was dark, the lights were off in the kitchen and she couldn't see where to step on. The broken glass was already on the floor, so I think Doug "prepared the environment" some time before. Remember, he used to enter her apartment when Amy was not there.
Although Doug might possibly have set the scene for her to have an accident, I'm fairly certain there was no glass on the ground when she first got home. Remember, she went straight to the bedroom because her dog didn't greet her at the door like he normally does. She would have had to have walked over the glass on the way to her bed room where the dog was sleeping on her bed. When she leaves the room, you hear her knock into something and then glass breaks.