This could be any of all the many things that could cause red
spotsBirthmarks - pigmented
Liver spots
Measles, koplik spots - close-up
Mongolian blue spots or irritation anywhere on the body, like acne, folliculitis, etc, etc. The only conceivable STD is genital herpes, which can occur on the buttocks--but initial genital herpes infections almost always involve the genitals, not the buttocks or other nearby areas. And if you had
recurrentRecurrent cystitis herpes, acquired sometime in the past, you would not have lesions on both cheeks; recurrent herpes is always limited to one side of the body.
Therefore, these spots are not an STD and have nothing to do with your sexual encounter a week ago. Of course see a health care provider if you remain concerned.
Good luck-- HHH, MD
But if you do have herpes, it is a recurrent outbreak and unrelated to your recent sexual encounter. Recurrent herpes episodes generally occur in the same spot each time; so see a provider promtly (within 1-2 days) if similar lesions occur in the future. But still, herpes is a lot less likely than other possible causes.
I told you above you don't have herpes and to see a provider if you were concerned. You did that and he confirmed you don't have herpes. So your new questions are irrelevant. And no, neck glands cannot swell as a result of a genital HSV infection. I already answered your question about the location of initial herpes lesions.
You don't say what blood test was done so I can't help there. You are now under a provider's care for this issue. This site is not a substitute for care by your personal provider, so that's the person to ask any further questions.
HHH, MD