I have black spot insde my mouth. what causes this?

Many reasons for it: It depends on how long you have had this black spot. Some of the pigmented spots are birth marks and you may have not noticed it before. But if it is new, we need to know its size and growth pattern. If it is growing in size or bleeds, it may need a biopsy done by a dentist or or an oral surgeon. But first you should show it to your family doctor and get his/her opinion..

Uncertain: This can't be diagnosed with the information that is available. A history, physical examination & other studies may be needed to determine cause/causes. Once accurately assessed a treatment plan can be developed.

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.