Hi Dear ,
Welcome to HCM.
Understanding your concern.
Thanks for your query . The symptoms you mention in query are pointing towards a condition known aphthous ulcer. There can be many reason for aphthous ulcer like stress and 
anxiety ,minor injury inside mouth ,
food allergy and  
vitamin deficiency . They can be rarely associated with disease like corhn's disease .
Minor form of ulcers do not required ant treatment ,they go away by there own .
I would suggest you to do 
chlorhexidine mouthwash rinses  , apply lidocain gel for pain , do warm salt water rinses several times day ,avoid hot and spicy food and  start taking 
vitamin B12  supplement 
I condition doesn't get better in 2-3 days consult dentist/oral pathologist for proper examination and to rule out any underlying cause.
Hope your concern has been resolved. 
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Get Well Soon.
Best Wishes,
Dr. Harry Maheshwari