For starters, pinworms arent really rare you know, there's more than 200 million ppl that has pinworms in the world, and a good fraction of it is in the us. They're not that harmful either, unless you're a female, then they can cause more damage.
Anyways, usually the eggs are laid outside of the anus, not under the skin. Because the way people are infected by pinworms is that an infected person will scratch their rectum and in turn will pick up some of those eggs under their nails, and then the eggs will be transferred to others from some sort of interaction between the infected person to the uninfected person.
So if it's bumps, perhaps it's skin tags that looks like bumps unless they're perfectly round.
Since you're not experincing any symptoms then you could almost eliminate that on your list, although many do not experience symptoms, but for most of the part you would experience some itchiness.
Another thing could just be that it's blood clots that forms under the skin in cases where you rub your anus too hard while cleaning.