Neil:
I'm not sure based on your description, but I think the spots you are asking about are blocked follicles, or folliculitis. This is a common problem, epsecially in the groin. I don't know why some people get it at certain times and not others, but treatment is difficult. Oral antibiotics help, but may not be worth it, since the spots go away anyway, as you note. You might try showering with Hibiclens, an over-the-counter antiseptic. If that fails, I suggest you show the spots to a doctor when the spots are in full bloom.
Best.
Dr. Rockoff
I used to have sweat problems myself. Look for a product called Certain Dry. It is a roll on and you put it on at night when your pits are clean and not freshly shaven (for females) because they will get irritated. I get mine at a drug store but it is a hard product to find so search around. Trust me it worked for me and a few of my friends too.