I understand you are not sexually active & dont suspect any STD
The lump under the labia can be one of many things but is most likely to be an infected cyst
of some sort.
Most likely a bartholins cyst
It will most likely require a visit to the gynecologist or dermatologist
, get a course of antibiotics & antiinflammatory agents that require a prescription & it should subside denovo. If not, it will require an incision & drainage
that your doctor will decide for you on visualisation.
Cheers & wish you good health!
Dr Praaveen Rodrigues MD