BASED on the information if it is "SEBACEOUS CYST" here are some things to know about it, their are few complication that can occur and may need attention, but it mostly an issues of cosmetic look for most of the people.
The complication that can occur are as follow
1) Inflammation of cysts can become tender and swollen, even if they're not infected. Inflamed cysts are difficult to remove.
2) Rupture of cyst often leads to a boil-like abscess
and needs a instantaneous care.
3) Infection of cyst can become infected spontaneously or after a rupture.
4) RARELY it can get converted to BASAL CELL CANCER
/ SQUAMOUS CELL CANCER.
Here is some information about sebaceous cyst.
Let me explain what do I mean, their are glands(bunch of cells modified to do certain things) called sebaceous glands which secrete the sweat and an oily substance called sebum, they have an opening at one end and closed at other like a bottle normally it can get the production out from the opening but due to one or other reason the opening is closed/sealed and hence the production of sweat / sebum can't come out so it starts building up and it seems like a bump still at this time it will be painless, but slowly it might get infected and the infection makes it painful which is present in your case.
May consider its removal if it keeps getting bigger and bigger, more over it protect form above mentioned complications also.
Hope it helps.