Hi
We have noted your concern, knowledge and care towards your oral health
Firstly I would like to bring to your notice that any material placed in mouth is treated as a foreign body if it is not compatible with your oral cavity. Not only amalgam, other type of Fillings/ materials cause irritation if they are not compatible
By seeing the picture the redness is spread over the middle of the cheek at the level where opposing tooth clench each other, so it might be traumatic in orign
Next Steps
follow up the size of the redness, if it increases and cause ulcer u need to consult dentist. otherwise it's nothing to worry