So I Was On Birth Control And My Fiancé Came

So I was on birth control and my fiancé came home on leave from the marines for 4 days and we decided to try for a baby I had just finished my period 4 days prior to him coming home so I stopped taking my pills we had unprotected sex multiple times while he was home and I started bleeding again today why would this happen and does that mean we definitely are not pregnant?