non- placebo pill and period

steph
It's my first month on the pill and I'm a little confused so hoping this isn't too dumb a question! So I was due to start taking sugar pills on Sunday but I chose not to and kept taking the real ones. Monday night my period came anyway which I thought was impossible so I'm a bit confused. Decided my body must know best and have started taking the sugar ones now but just wanting to know if anyone can explain it to me! 
Edit: Also started antibiotics on Monday could this have had an effect?