Welcome to Android Central! How hot? Painfully so? Is the battery draining quickly? It's normal for phones to get pretty warm if you're doing a lot with them, like installing lots of apps or running a very processor-intensive app (usually a game).
Poor cell signal can cause the phone to heat up, as the phone tries to boost signal reception. A corrupt file on the SD card can also cause heat, since the system's Media Scanner gets stuck on that file, leading to excess CPU usage and therefore increased heat production (and battery drain).