What Causes Bleeding After Depo Injection?

i had my first depo jab in jan 8th i have bled 3 times since the first 2 for 2 weeks at a time and the last time lasted for 3-4 weeks now im off im conserned i may be pregnant every time i come off i use it as a chance to sleep with my boyfriend but the bleeding starts again strait after the doctor gave me tranemic acid but it didnt work can you help