This could be herpies. Due to transmission to other partners, I would have it checked with your Dr asap. I would not have sex until I found out, or at least unprotected sex or while leasions are there.
It's hard to say without knowing exactly where the lump is located in/on the vagina. Is it high up on the labia or very low?
If it is low and situated near the opening of the vagina, it could possibly be a Bartholin's cyst.
This is formed when one of the Bartholin's glands gets blocked, and is usually not an infection simply a blockage of the gland but it is very painful.
I'd suggest you see a doctor as first, you don't know that it IS a Bartholin's cyst and if it IS, depending on the severity of it, it may need to be drained or could require antibiotics.
In the short term, regardless of how the lump has been caused DO NOT try to squeeze it or pop it.
Try warm sitz baths or hot compresses to relieve the pain, along with normal pain relief.
Hope you start feeling better soon :)