Is mouth pain from cavities or something else?

Too many possible: Sure a cavity can cause pain, but it could be a sore in the mouth, an infection, a bone cyst in the upper or lower jaw, a bruise from trauma, an extra growing tooth, a deep abscess needing a root canal. Best to see a dentist and get it checked thoroughly.

Possibly: Tooth decay (cavities) can definitely cause pain. But one can also have pain from infections of the gums and salivary glands, cracked teeth, lost fillings, trauma, and cancer.

Perhaps: Perhaps. Many reason for mouth pain. See dentist or oral surgeon for exam.

Hard to say...: ... without an examination. Your question is too broad with too many potential answers.