you will need to see your health care provider or gynecologist for a proper examination to determine the true nature of this "lump". your description is just too vague for me to be able to narrow down the several possibilities but one common lump could be a bartholin gland cyst. you can do a little research to see if that fits the bill. the fact that this lump is bigger and more painful emphasizes the importance of medical intervention.
i have a lump on my vagina that came back after it went away for a week it came back alot more painful and alot bigger