What causes small bumps on the back?

Uncertain: There are a # of possible causes. The best way to get an accurate diagnosis would be dermatological evaluation.

Visual DX: Hi, it’s hard to say without a picture but there is a great app that may be able to help you get a diagnosis without being seen in person by a dermatologist. The app is called VisualDX and is one of the best, if not the best out there. Many clinicians, including myself, use it. Just snap a photo and it gives you a list of possible diagnoses. Good luck!