check the battery usage of the applications:

a. charge the phone to 100%

b. use it with no charging in normal use

c. once it get hot and no application is running open phone settings and select battery 

d. on the upper right side of the screen click the 3 dots and select battery usage

e. check which application consume a lot of battery - this might be the problematic application. kill it and check the device again after cooling it down 

2. make sure the device is not in hot place or exposed to direct sun for too long 

3. Some phone covers does not let the device cool - replace cover and check again