I think allergy is extremely unlikely to be honest with you.
When you shine a light on the roof of your moth and look in the mirror what, if anything, do you see?
Here are a few possibilities:
(1) I would not be alarmed by this mild infection on your soft palate. It would be OK to let a few days go by and to see if resolves itself.
(2) Fungal infection is a real possibility.
(3) If you are not getting better in a few days then you will need to see a doctor. You can't diagnose something like this over the Internet.