Unfortunately, herpes is quite contagious right before and during the time that a lesion appears, so you’re right, there is a good chance you’ve already contracted the virus if your partner kissed you or performed oral sex on you right before the sore appeared. But, there’s actually a good chance you had it anyways, even before that (and in that case, you would not catch it again, things would just stay as they were). Many people contracted herpes on the lips as children, from parents or friends giving them kisses on the face. We think that about 75% of Canadians have either oral or genital herpes (either type 1 or type 2), so it’s more common to have the virus than not to. The good news is that although herpes can be uncomfortable and annoying, it is usually not dangerous in any way to have it.
Hope that helps answer your question,