What Causes Prolonged Period Along With Abdominal Cramps While On Depo Shot?

Hello. I have been on the depo injection for about 10 years now. I have had very very sporadic mini bleeds (maybe 2 a year). However during the last cycle of depo I had a 2 day period and during this cycle I have had a longer period and bad cramping. I am just checking this is just random and not a sign that it is stopping working, or my body is reacting differently to it? Many thanks for your help