Small pimple like bump on anus?

Need more info: If you are sexually active you need to be tested. Otherwise what looks like a pimple, probably is and would be mildly tender. It would also more likely be adjacent to the anus as opposed to right on it and would erupt and resolve over a week or two. If it's nontender and solid its more likely a wart such as HPV. See a doc if its there longer than a month. Warm compresses or sitz baths would help.

Need to examine: It is not feasible to provide a meaningful opinion without examining you. It may be as simple as hemorrhoids to as serious as anal carcinoma. It would be prudent to consult your doctor.