Only a viral culture will really give you the information you seek. Short of that, the blood test may well help to sort things out. Let me give you a little information which may be helpful, either now and even more so when your blood test comes back.
HSV does relatively commonly occur on the buttocks and when it does, the lesions resemble herpes zoster (shingles) and are commonly misidentified as such. The lesions usually appear as a cluster rather than a more elongated rash. If your lesions responded rapidly to valacyclovir, that even makes HSV more likely. If these lesions were herpes, your blood test will most likely be positive.
As for shingles, it is rather uncommon in persons under the age of 40, occurs but rarely, and when it occurs, it usually covers a stretch of skin rather than occurring in a cluster. Herpes zoster is very uncommon on the genitals.
Finally, if you experience more lesions, either on your buttocks or your genitals, see your gynecologist and request a culture as soon as possible after the outbreak occurs--this is really the best way to sort things out.
Hope this helps. Please let us know what your blood test shows. EWH