She was masturbating and she said her clitoris becomes painful, what might be the cause of this? she is new to masturbating. is this normal?

Irritation: If she is new, perhaps it was overdone, done with poor technique, lack of lubrication or maybe there is some other cause for irritation. Could be normal or expected for a beginner. Perhaps needs some friendly advise or some good reading material. Purchase a good lubricant. Wait a while for things to settle down before resuming this activity. If in doubt, get things checked out by your doc.

Masturbation: Probably did it too hard. Ease off.