Hey there,
Your Apple lithium-ion battery uses fast charging to quickly reach 80% of its capacity, then switches to slower trickle charging. The amount of time it takes to reach that first 80% will vary depending on your settings and which device you’re charging. Software may limit charging above 80% when the recommended battery temperatures are exceeded. This combined process not only lets you get out and about sooner, it also extends the lifespan of your battery.
as you say maybe your charger is not original if you bought it from a third party store make sure to buy any chargers to your Apple devices from Apple Store or AASP (Apple Authorized Service provider) you can find one by this link ☞ Find Locations - Apple
if you bought this charger from Apple Store or AASP see this article ☞ If your iPhone, iPad, or iPod touch gets too hot or too cold - Apple Support
this is all the normal situation that will make your iPhone is warm:
When you use the device or charge the battery, the device might get warm. You might notice that your device feels warmer in these situations:
When you set up your device the first time
When you restore from a backup
When apps reindex or reanalyze data, like Photos tagging for faces, places, or keywords after a software update
When you use graphics-intensive or augmented-reality apps or features
These conditions are normal, and your device will return to a regular temperature when complete.
Here are some of the higher ambient-temperature conditions and activities that might cause the device to change performance and behavior:
Leaving the device in a car on a hot day.
Leaving the device in direct sunlight for an extended period of time.
Using certain features in hot conditions or direct sunlight for an extended period of time, such as GPS tracking or navigation in a car, playing a graphics-intensive game, or using augmented-reality apps."
