Why does the volume on my iPhone randomly keep turning itself up?

[Lucy]

For some reason, no matter what I'm doing on my iPhone (I might just be looking on Facebook), the volume will sometimes very randomly suddenly turn itself all the way up. It's really weird. Sometimes I can just turn it down again, but sometimes it's even worse and will keep turning itself up when I'm trying to turn it down! It's really annoying and I have no idea why it does it, there's no reason for it. It's nothing to do with my case either because it did it before I got a case for it as well. I've only had it for a year. Why might it do this? Is there any way of fixing it? Should I take it to the Apple Store?