even the best blood tests we have, still miss 1 out of every 10 hsv1 infections. Best thing to do is to be seen when you have your oral symptoms promptly and get a lesion culture done of them.
grace
If you get tested within 6 weeks of possible first exposure to hsv, your blood test may return a false negative and so your lip sores could be herpes and your test may not be detecting antibodies. If you have tested atleast 6 weeks after the possible exposure to hsv and your blood test(at least the herpeselect type specific igg test) is negative then you can be fairly certain you don't have hsv. The false negative rate is 3 in a 100 I think. The gold standard would be the western blot.
It's most likely a pimple? If it ever has white puss then it's a pimple. If it ulcerates clear liquid on it's own and scabs up (usually yellowish scabs) then it's HSV. Also, the time frame of pimples is usually much shorter than HSV.