period 10 days early?

my boyfriend & i have had unprotected sex the past couple weeks, though we've only had sex like 3 or 4 times. each time he's pulled out in time & didn't cum immediately after. i started bleeding about 30 - 45 minutes ago. after looking it up on Google, i read it could be implantation bleeding. i really. really. really. cannot be pregnant. yes, i should have been thinking and told him to wrap it up but this hasn't happened throughout these past 5 months of us dating. is there any other possible reasons for me bleeding this early.?