Gentle stretches can help, sometimes -- check especially to see if the muscles that run on either side of your spine are tight. If they are, see if you can either stretch them out gently, or get someone to give you a good backrub.
Warm or hot baths can be really helpful too, and some women find that masturbating to orgasm is a good way to relieve cramps at least temporarily. Deep breathing and relaxation exercises are also also a good thing to do whenever you're in pain, as you'll hurt more if you're tense.
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