This looks like a traumatic ulcer to me.Before I can answer to your queries I would need some additional information.
Do you often bite on your cheeks?
Did you have any medication recently?
Do you have exams or any kind of stress related events coming up?
Do you often have the kind of ulcer especially during periods?
Next Steps
Don't panic this will heal on its own or with medication.
Health Tips
Please get this evaluated by a nearby dentist for long term benifits and root cause elimination.