I've read that masturbation helps to reduce menstrual pain. Is it true?
Only indirectly. Orgasm, which masturbation can trigger, does release chemicals which have a pain relieving effect on the body. The problem is that these chemicals do not last very long and the pain from the cramps return. An over-the-counter pain reliever works better and lasts longer.
Masturbation stimulates the same muscular contractions as present in menstrual cramps. A few women find that masturbation actually makes them feel worse because of this.