Hello Lily and thanks for the query
As per your symptoms it seems to be a cyst
but we can not forget lymph
nodes if it at inguinal region. The causes may be infection, inflammation or growth of the surrounding tissue. The treatment depends on the causes of the lump.
To find out the cause consult your doctor and by clinical examination and investigations like Fine needle aspiration test (FNAC) or scan she can get to a diagnosis. Treatment can be planned their after.
I don't think that it is boil or ingrown hair because they are painful on touch and increase with time.
It is better to consult your doctor and get clinically eamined.