Late period

Me and my partner have been having protected sex this month. One night we didn't. He just pulled out. I'm normally bang on with my cycle every 28 days. I took a pregnancy test at day 30 and is negative. I'm 4 days late in period tomo. I can't be pregnant can I? I've been getting white discharge sorry tmi. It don't matter if I'm preg as we are gonna start trying for our 3rd soon. I just don't understand it x