How worried should I be about this?

I was home alone last night. The only people that have keys to our apartment are me, my mom, her friend (whose house she slept over at last night), and the landlord. So I went to the park last night. Got home around 10, I distinctly remember locking the door. Went to bed arouund 3:30, the door was still locked. When I woke up at 11, I saw that the door was unlocked. It was obviously still chained, I have small hands and even I can't unchain it from outside. But now I'm pretty creeped out. Am I worrying too much? I'm home alone a lot, not to mention I'm a weak teenage girl lol. We have a gun but it's in a safe and my mom refuses to tell me the code even though I'm responsible. Should I just forget about this or what?