Started BC on late period

Hey y’all. SO the last time I had sex was 6/29, and I got my period in July. I was supposed to start my last period this past Saturday, it’s Wednesday and it hasn’t come. I usually have pretty regular periods, except I had to take a Plan B in February, so that messed my cycle up a bit. I was planning on starting my first Birth Control pill on Sunday, but my period didn’t come. I got impatient and started today. Should I have? Any ideas as to why my period is weird?? (s/n, I recently moved into college so i’m thinking it may be stress related)