Hi, thanks for your query.
You seem to have developed folliculitis
or boil on your butt cheek.
Pain indicates collection of pus.
It is caused by sweat, moisture and friction, and even constant sitting.
To prevent this in future, you may do as follows:
-maintain proper hygiene
-use syndet based cleansers as they maintain skin pH
-wear loose fitting cotton clothes
-avoid constant sitting for long hours.Take a break and move about a few minutes.
Hope this answers your query.