iPhone SE gets quite hot after replacing the protector casing

Use iOS devices where the ambient temperature is between 0º and 35º C (32º to 95º F).

^{source: https://support.apple.com/en-us/HT201678}

Some of the reasons I can think of:

• Not having proper ventilation around the phone. Even though this is a prominent thing for laptops, phones also need to dissipate heat.

• Low cellular connectivity. Poor Wi-Fi won't heat up the iPhone much, but cellular is the main culprit if it's poor. (personal experience)

• When you use graphics-intensive or augmented-reality apps or features.

  Which you're doing. Camera also falls under this(personal experience).

If device overheats, some features will underperform, like flash or camera, dimming of display and other graphic intensive tasks. It might even get locked if temperature goes beyond a threshold.