Thanks for the query.
The possible causes for your lump are:
which occur due to infection of hair follicles.
Clogged sweat glands can form as lump and when they get infected presents as painful lump.
Infections like warts, herpes, molluscum contagiosum
etc cause lump on labia majora
Any type of infection should not be neglected in pregnancy
as that can affect the well being of the baby better to consult gynecologist and get examined to find out the actual nature of the lesion.
Depending on that treatment can be taken.