You may need to see a doctor for this. Sometimes women and girls get Bartholin cysts - it's a special type of glandular cyst that won't clear up completely without medical treatment, even if it bursts. They do hurt - and they aren't related to sexual activity at all.
Here's a link to the Mayo clinic's page on these Bartholin cysts: http://www.mayoclinic.com/health/bartholin-cyst/DS00667
In the meantime, my doctor told me that warm (not hot) baths and warm washcloth soaks would help until we could schedule the (minor) surgery to remove it. She also recommended acetaminophen for pain - I believe that's paracetamol on your side of the pond.
It's nothing to be ashamed of, whether it's a cyst or an infection of sorts, so tell your Mom or an auntie or just get yourself to your GP if you can. Sorry about your holiday plans - I hope you aren't too miserable from this. Good luck to you - let us know how it turns out. Warm regards.
Try sitting in a hot bath. It sounds like yo have an ingrown hair or boil. Try popping it after you have sat in the water. It will hurt but it may also solve your problem. If this is what it is, you may have to continue expressing it until it clears up. Whatever you do....do not use peroxide on it!
Soak three times a day with a warm washcloth (it is a nice touch to have it be a salt-water soak) and then when it bursts and drains (don't force this! no squeezing), keep it covered with a coat of Neosporin or Polysporin. Or, see a doctor. I'd see a doctor in a few days if it doesn't open, in any case.
It's probably a cyst or a boil or even and ingrown hair that has gotten infected. I usually once in a while get a large lump from an ingrown hair. It's really painfull. I put warm compreses and help it go down. Hope this helps and I think you should go to your doctors.