Sometimes they are a constant dull ache. The discomfort may spread to the hips, the lower back, and the inner thighs. The vagina or the uterus may ache and feel heavy. Sometimes the cramps are very severe and are associated with nausea, vomiting and diarrhea. There can be generalised body aches along with the pain.
Menstruation is supposed to be a natural event.
The uterus is a muscle. Like all muscles, it contracts and relaxes. Most uterine contractions aren't even noticed, but strong ones are painful. During menstruation, the uterus contracts more strongly than at other times of the month. This can produce the uncomfortable feeling of menstrual cramps.