Birth Control problems!?

So, i took my birth control shot (Depo-Provera) in late February, and in mid may i needed to go back to take it again, but i decided not to. i got my first period june 3 and it lasted around 4 days. that's the usual time for me. well i had sex about a week ago and i bled very little. i was a bit worried, then i ended up getting my period a couple days later. do you think i bled because of the shot? or could it be something else? i had though maybe an STD? but i highly doubt it. Help please!