My iPhone 5 won't charge when plugged into the wall?

I've had it about 5 months now and I've never had a problem with it.

Ever since last week, the battery has started to drain faster than normal and my phone won't charge when I plug it into an outlet. I've tried every outlet in my house and classrooms. I have to plug it in different ways or push it the plug hard into the outlet and then wait a few minutes for it to start charging and even then sometimes it won't charge. I've tried resetting it and then plugging it back in to charge but that hasn't worked either.

At night I keep my phone next to my bed and it's plugged into the outlet behind my bed.

I don't think something is broken INSIDE the iPhone considering I've taken good care of it, so could it be that the lightning cable is faulty? Or something else?