Hello and welcome to hcm forum,
is caused due to infection of tonsils.
I would advice you to get a clinical evaluation by an ENT specialist
After all,tonsillitis is not associated with allergy from retainers.
But,the ulcers may be developed due to use of retainers.
Please discontinue wearing retainers.
Maintain oral hygiene
Plenty of fluids and vitamin rich diet has to be taken.
Vitamin becosules,iron and folic acid
supplements has to be taken.
Avoid stressful works.
Hope this helps.