Depo Pervero

if you are on the shot as your birth control, please answer me question. my doctor told me that a lot of girls spot in the first month, as a replacement for your period. well i have an irregular period, but i was taking the pill for a while before i changed to depo, so my period was regular. i got my first shot in December of 2016. i spotted for the whole first month, maybe a few more days. has this happened to anyone else?