What Causes Muscle Cramps After Taking Depo Injection?

I have been on the depo injection for about 2 years .. then i missed my oppointment for my injection for amounth due to a death in the family..when i went to my doctors he asked meto takea test to see if i was pregnant but i refused as i felt fine and had no sex for a mounthbefore i was dueto have the shot again..but now i have had the shot i am having severe muscle cramps every hour for five minits why is this?