Welcome to the STD forum.
I'm not sure why you suspect your problem might be an STD. From your description, it sounds possible that you have oral herpes -- but that isn't normally sexually acquired and is not considered an STD.
As for your statement "I'm quite certain this is a cold sore": Cold sores usually are more superficial than pimples, and usually there would be more than a single lesion -- usually a cluster of a few blisters in a group. Still, I agree a cold sore is possible. It would be helpful to know whether your tests for "all STDs" two years ago included a blood test for HSV-1. If not, you could have oral herpes that goes back to a distant past infection, perhaps in childhood. (This is possible even if you had a negative HSV-1 test. The HSV-1 blood test is not very accurate and misses lots of infections.) Finally, even if you have new oral herpes, there is no reason to suspect it was sexually acquired. (OK, maybe kissing is a form of sex. But diseases transmitted by kissing are not considered STDs.)
If you have oral herpes, transmission to anyone else is unlikely if you're not having another outbreak. Unlike genital herpes due to HSV-2, asymptomatic viral shedding of oral HSV-1 is fairly uncommon and most infections are transmitted during overt outbreaks.
Is your boyfriend at risk from kissing you? If he is the person you caught it from, then definitely not: people are resistant to new infections with the same HSV type, and couples do not "ping pong" their herpes infections back and forth. If you caught it somewhere else, he might be at risk -- but there's a 50% chance he has had HSV-1 himself, in which case he also is resistant.
Bottom line: If and when a similar outbreak appears, see a health care provider within 1-2 days for professional examination and perhaps testing for HSV-1. You could also have an HSV-1 blood test -- but even if positive, it won't mean that herpes was the explanation for the sore on your lip. In the meantime, to be safe, avoid kissing anyone when you're having a similar outbreak. Otherwise don't worry about it.
I hope this helps. Best wishes-- HHH, MD
No. People never get HSV except by close personal contact with infected persons, or the overt exchange of body secretions.
Thanksk so much!! Your input helps. I thought of one other thing... is it possible to transmit through sharing food (food specifically, and not eating utensils)?