|(NOT A MEDICAL DOCTOR)
Looks like the scratching is causing more "infection" than it is new bumps. are you waking up in the AM with more bumps in new places? If so, Check ALLLLLLLLL things you were wearing. Rings, Watches are bad for it, so are Necklaces and bracelets. I know from experience that if you scratch and scratch and scratch more you WILL slow healing time and get your little bumps infected with the pus again.
For your left arm: SARAN WRAP! I've tried it once and it worked really well on my left calf. Wear a long-sleeve shirt and tape on some saran wrap with bananas in it. It will help.
(NOT A MEDICAL DOCTOR)