Initial herpes generally occurs on the genitals, not the buttocks--even though buttock or thigh lesions sometimes occur as recurrent herpes. However, the pimple you describe doesn't sound like herpes. (Think of herpes lesions being superficial, i.e. "on" the skin, whereas pimples are deeper, "in" the skin.)
NGU rarely is due to herpes. When it is, usually there also are classical herpes lesions on the penis at the same time. Also, the NGU with herpes tends to be severe, with lots of pain, especially with urination. Testicular and groin pain are not signs of STD, unless there is overt swelling and marked tenderness to the touch. And groin or testicular pain alone is not a sign of herpes.
I don't know how much your clinic or the specialist you saw knows about STD. If you were in a public STD clinic (or GUM clinic in the UK), you can rely on that person's expertise. The things she told you sound pretty accurate to me, as best I can judge from your descritpion. Most likely you can rely on her advice about your diagnosis and further testing--but it seems very unlikely to me that you caught herpes during the exposure you describe.
See many other threads about the virtually zero risk you caught HIV. Search the STD forum threads for "HIV transmission risk" and "HIV anxiety"; you will find 50 or more questions on this, most of them more or less like yours.
Good luck-- HHH, MD