iPhone SE gets quite hot after replacing the protector casing

I recently bought a new iPhone SE case, a silicone one in book style from B2CTelecom. The particular one I bought in case it matters is https://www.bol.com/nl/p/iphone-se-5s-5-standcase-hoesje-design-stars/9200000104229975/ I've found that while watching online videos like Youtube and Twitch, it gets quite hot after about half an hour to an hour. is this because of the new case acting too much as an insulator? Could this end up damaging my iPhone? The iPhone was bought new in 2016, and the battery max capacity right now is at 91% according to the settings app.