For menstrual cramps, a heating pad really works. If you don't have one, a ziplock bag, some paper towels, and a hotpot will work just fine. Boil some water, soak the paper towels, put them in the bag, and wrap a paper towel around the bag. Be careful not to burn yourself.
Another good way to do this is if you're making tea, take the teabags and put them in the bag, and wrap with a paper towel.