Do i need to have a period after injection to be pregnant

Patient: After bein on the contraceptive injection since my son was 4 months old, hes now 3. my last injection was in august. i have had sore boobs, heartburn, stomach cramps, goin to the toilet a lot and have even been sick. ive done 3 pregnancy tests and they have been negative. i havnt had a period since coming off the injection. could i be pregnant?