One of the most common reasons for your cell phone becoming hot is a malfunctioning charger. If you notice the cell phone becomes hot during or just after charging, this is the most likely culprit. Other reasons include using the wrong kind of charger, insufficient air circulation and heavy application usage.
Malfunctioning and Improper Charger
Malfunctioning or broken chargers are the No. 1 cause of cell phones becoming hotter than usual. Broken chargers typically channel the wrong voltage to your phone's battery, causing the unit to heat as the excess electrical energy transforms to heat. You should also be aware that older phones use nickel-based batteries, while newer phones use lithium-ion batteries. Using a charger that is not rated for your battery type can also cause your phone to heat just as if you were using a malfunctioning charger.
Insufficient Air Circulation
Computer processors generate heat. Cell phones use computer processors, and are designed to take that heat from the processor and transfer it somewhere where it won't damage the sensitive internal electronics: typically to the phone's external casing where the excess is disbursed by normal air circulation. Keeping your phone in your pocket or in any similarly closed location prevents the air from cooling the phone.
Computer processors generate more heat the more they are used. If you are using your phone for extended periods of time, especially for process-heavy applications, you should expect the unit to heat up more than usual.