Greetings,
Painless hard lump of small size (as can be felt by finger but can't be seen) p/h/o tobbaco user - all this suggest to rule out oral cancer. Although you stopped taking tobbaco.
Other causes can be possible for this painless solitary mouth ulcer.
Next Steps
Go for FNAC - ENT or OMFS/dental surgeon.
Health Tips
Don't be panic.
Proper diagnosis and management on time will be helpful.
Start taking anti - oxidants like VIT C or VIT E. or cap. Coenzyme 100mg once a day for supportive treatment.