Thanks for asking the query,
Cheek piercing had caused a trauma resulting in the accumulation of healing tissues and cells in the area resulting in the formation of cyst or little beningn fibroma
I would encourage you to stop such practices, if the irritation continues,in long standing cases this lesion can develop into malignancies.
I would suggest you to get a clinical evaluation done by a Dentist
Maintain a good oral hygiene
Hope this helps out.