Birth Control and Periods

So two weeks ago I went to the doctor to get on birth control, he of course did a pregnancy test (came out negative) but did not start birth control (idk why).

Then the last week I went in and got the birth control shot. Two Days before my period should have started.

My period is now two days late (three if it doesn’t start today) and I’m not sure why it is.

The last time I was sexually active was at the beginning of January. He pulled out, I know not the best way to go about any of it.

Could the late period be because of birth control? Or? And what should I expect while on birth control?

The doctor only said that it could make it to where I don’t have periods and that once off it could take a couple months to little over a year to start having one again.

I’m just here to get advice. Not be lectured about my choices or anything.