This is going to be long, but I really hope it helps you understand it more, and makes you feel more at ease. I feel like I'm experienced enough in this area to give a detailed comment.
That's a coldsore then. So she had a coldsore when she kissed you? You would've noticed that straight away, but if you didn't, you could've caught it anywhere. Symptoms can lay dominant for years, and then flare. Coldsores and genital herpes are easy to treat.
I caught a coldsore when I was about 10 (like a lot of children do), and I had no flares until I turned 30. I started getting flares every few months after that. And for the last 2 years, I've been getting them monthly, and bad, but I have a compromised immune system due to an auto immune disease. Some people get them every 2 weeks.
You can only pass it on if you kiss someone or share the same utensils etc, and in the first stages of the flare. You would 100% know if you had herpes. It is blisters.
Some things can look like herpes, but could actually be something else - like impetigo (skin infection) or another skin infection. Recently, I thought I was having yet another flare, but turns out, someone I was seeing gave me impetigo (but he had no symptoms, weird).
I have genital herpes, I caught it about 12 years ago, when I stupidly believed a bloke when he said he had a latex allergy, and he told me what was on his dick, was from the latex allergy - turns out it was Herpes, and he didn't bother to get tested, just passed it around basically (gross!). I have never passed it on, not once, and I always tell sexual partners that I have it. I've had a total of 3 flares since. It isn't as bad as you think.
Herpes is VERY common. A huge portion of the population have it, and don't even know it, because it can lay dormant for years, decades even! They used to test everyone in the sex clinics for it, but they don't now, because it is THAT common (a sexual health nurse told me this). It is VERY EASILY treatable, and nothing to be ashamed of.
Honestly, it's nothing to worry about. Not pleasant, of course, but nothing worth killing yourself over.
I would recommend going to the doctors.
I hope this helps
Best of luck