I know this post is now a couple years old, but if by any chance you are still dealing with this, I think I figured it out. I have had this issue since I've had tattoos!!!
I have trouble with candida and have always had bumps on the backs of my arms, but I have some that never go away inside healed tattoos and they flare during the summer months.
I just tried dandruff shampoo Nizoral for my scalp, and figured I'd try it on my arms and accidentally discovered these bumps magically going away! My tattoos are brighter, almost new looking again too.
I think your bumps are yeast spots/candida! It's totally normal, but my guess is postpartum hormones spiked an imbalance/overgrowth for you, which I hope was temporary. If not- try this!