Thanks for the query.
According to your description, the lump on your vagina could be a Bartholin cyst
. Bartholin gland
lies near the vaginal opening.
Its duct opens through the vaginal lip.
When the duct of the gland gets blocked, cyst will form.
When the cyst will get infected, abscess
So you once consult gynecologist, get examined to find out the exact cause of the problem.
After that with her advice, you can go for antibiotic course.
While prescribing the antibiotics, mention that you are breast feeding the baby.
So that she can prescribe the drugs that can be used in lactation