A knock on the door

I remember something that happened to me, not exactly a scary one, but it gave me quite a fright.

My mother and I live in a one-room apartment together. My mother works all day, and I'm home. Our front door is loud—when you open it, the noise is loud enough to be heard from the kitchen with cotton wool in your ears. I was in the bathroom that day. I went in, locked the door out of habit, and started washing my hair. I soaped it up, started to wet it, and then I heard a distinct knock on the bathroom door. I pricked my ears—I didn't seem to hear the front door opening. I wondered if I'd misheard it. I stood there for a minute, frozen, and then calmed down: maybe it was the neighbors knocking upstairs, or maybe I'd just heard it. I washed my hair, grabbed the hairdryer, and there was another knock on the door. I actually saw the door shake slightly from the knocking. I assumed my mother had returned and I simply hadn't heard her. I opened the door—it was empty. Confused, I called out, "Mom? Are you home?" Only silence answered... That's when I got scared. I quickly latched the door and started drying my hair. And then there was another gentle knock on the door. Needless to say, I was stunned. I sat in the bathroom all day until my mother came home from work. Then I came to my senses.

I still sometimes can't believe what happened, but I clearly saw the door shake when the knocking happened—which means someone really was knocking on it. But who, I don't really want to know...