Macbook Pro 2015 continuously powered on
This is in almost all cases caused by the battery swelling up and thus "bending" the case. The case expansion cannot in itself be caused by "overuse of fans".

You need to have your battery checked and replaced - don't wait with it. It is caused by a defective battery that swells up, and potentially could lead to a dangerous situation (such as causing a fire) if left unattended. This can happen not only when the computer is plugged in and powered - but also when the computer is turned off and not powered by an external power source!

Remember that the battery is a wearing part, and yours is probably 4+ years by now.