First, there was a bag on my arm, which itched and scratched, then it scabbed, and then it fell off and scabbed. Here's the thing. What is this?

According to the photo, your condition belongs to eczema. This is an allergic disease, which needs anti-allergic treatment, so we must avoid it.

Guiding opinions:

It is suggested that chlorpheniramine or loratadine tablets should be taken orally and Piyanping should be used externally. To fast spicy food and seafood, avoid alcohol and tobacco. Keep the stool unobstructed and prevent getting angry. Don't stay up late.