I have black spot insde my mouth what causes this?

See if it persists .: You need to show it to your family doctor or dentist. If it is more than a dime in size and if it is a pigmented lesion(black or brown in color) you should get it checked by your doctor or a dentist. If it goes away in 2-3 weeks, then you may not need to do anythjing further. But if it persists you should remember it can be a melanoma which will require a doctor exam and if suspicious a biopsy don.