Hard to be sure from the picture, but cold sores don't go away in a day. Also, if you catch herpes of lip, the first attack is much more severe--swelling, swollen glands, fever, and so forth. And the incubation period is a few days, not 15 weeks. So on many grounds, I don't think you need to worry. As to what exactly caused the blister, it's hard to say. If it recurs, have a doctor take a look.
Best.
Dr. Rockoff
Just a couple more more questions. Would a cold sore stay for less then a day? Plus, if it was a cold sore from this girl, wouldn't I have noticed it before 15 weeks after the exposure?
Thanks again!!