iPod touch 4g get warm at top

My iPod touch 4g 4.2.1 gets pretty warm on the top where the sleep/wake button is. This happens to me while doing my apps, and even sometimes my home screen. But what's weird about it is that it's random: its on and off the whole time I'm using it. Anyone els have this problem or can help me? Or is it a problem at all? Thanks.