It is possible that you have fever blister otherwise known as mouth ulcer. This is very contagious condition so avoid kissing your boyfriend, if you have one. Also this means that your immune system is not the strongest one so you should work on that and this will also help you to heal faster.
Unfortunately you will have this nasty blister on your lip for about 10 days but do not despair because you are lucky to have only one. Increase intake of fluids into your organism and especially the ones containing vitamins such as orange juice but avoid beverages containing caffeine and alcohol.
You should be feeling better in a few days and hopefully you will not see those nasty fever blisters again.
This is very problematic. I know that my close friend had problem with her lips as well. She always complained how she had dry lips and she was always biting her lips and one they she caused them to bleed even. After few days she had these blisters on her lip that stayed there for several days. I know that she used lip balm to keep the lips hydrated. I am not sure if she used anything else to get rid of the blisters. But the good thing from those blisters was that she never bitten her lips again. My suggestion would be to keep your lips well hydrated all the time and about touching them with dirty hands.