A lot of people get infected hair follicules simply by walking. I would definitely suggest getting it checked again if you are that worried.
Like any part of the body, your groin can get pimples. If you miss a shower you will naturally get fairly dirty.
If you ever get a marble-feeling pimple underneath your skin, that is called a sebaceous cyst. It is like a huge pimple. They do not go away, really, and can leave scars. Sometimes they require surgical removal.
This isn't really a rule of thumb, but I think that if sebum comes out of something, then it is a zit.

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