iPhone voice command out of control?

I've only had an iPhone 5S since February or March and it's already bugging out on me. Sometimes Siri pops up randomly, as if I'm speaking to it. I'll be standing a room a way from it, not touching it in any way, and I'll see the screen go black and those little voice waves appear. I went to settings and turned Siri off, but now it just does this weird voice command thing where it will start listing all the options, suggesting things I can do with the phone, and it will almost always end with automatically calling a random person from my contacts list. It just dialed my dad's girlfriend two minutes ago and that's the second person it's randomly tried to call today. It's developed such a mind of its own that I'm afraid to leave it alone for even three minutes, lest it call a crush or employer or worse.

Anyone have any idea what might be happening here and how I might resolve it?

Thanks in advance.