When can I have unprotected sex

I got my 1st depo shot two days after my period on the 21st can I have unprotected sex now ? It's the 29th. My doctor confused me saying I should wait a month but I told him I heard it would work right away and he was like let me go check it out then came back and said "oh yea maybe wait a week " and left I'm really confused. ..any advice would be appreciated thank you