I am so sorry. I get these once in a blue moon. They spread and hurt too.
I learned when I was in college what to do, and I hope you will go to the drugstore NOW and get this so that you will start feeling better soon.
Get some Campho-phenique. I promise you that not only will it take the heat out in a day, it will stop the spread.
Since you have a bad case of it, get the bottle and apply with a cotton ball. Also get the tube to carry around with you. Use it often the first couple of days - every time you feel the heat. After it gets a little better, around the second day or so, I put mentholatum Chapstick on my lips in between campho-phenique applications because the campho-phenique dries them out so much.
Ever since I started doing this, any blister I've gotten has been short-lived and easy to deal with. As SOON as I feel one coming, I pull out my little tube, apply all day long, and that's pretty much it. The key is to start right away.
So get to the drugstore!
All my best,
Dawn