These could probably be a few different things - infected hair follicles, dermatitis, or even a reaction to your shampoo (your scalp may be too moisturized, and you might be getting clogged oil glands, for example).
https://www.healthline.com/health/bump-on-back-of-head#causes
https://www.webmd.com/skin-problems-and-treatments/sores-head-scalp#1
Really, though, the only way to know for sure is to have a doctor check it out. Stop picking at them, and let the doc see them as they are, in case they want to swab or test them somehow. Let us know what happens!