What Causes Pregnancy Like Symptoms While On Depo Shot?

My girlfriend is on the Depo birth control shot. She has been on the shot for about a year now and she regularly gets another shot after the three months is up. We are having sex ocasionally but lately she has been experiencing the syntoms of being pregnant like cramps, upset stomach and headache. I worried that she could possible be pregnant but we are unsure because she has never experenced these syntoms before. This is the first time in 4 month that she an i have had sex. Do you have any idea's what this could be?