Why Does One Have Cramps Before Menstruation?

Hi, may I answer your health queries right now ? Please type your query here...hi i missed my appointment get get my depo jab on 10th dec 2 weeks later i had what felt like cramp before period starts then it just disappeared with no period and now as time has moved onwith still no period i feel sick all the time and tired i took pregnanv=y test and it was negative is this normal is my body just adjusting as i am used to having depo injection