I had sex the day after my most fertile day later in the night. We didn't use a condom and he came in me. Could it be possible that I could be pregnant since it was the day after my most fertile day?

Sure you could be. Are you? You'll have to wait to find out.

Of course, you can get pregnant on your less fertile day, the odds are just worst.

Also how do you know it is your most fertile day. Are you tracking it with body temp ?
Those online assumption charts that tell you it is 14 days from last period are correct for some people, some of the time.

The fact is, people get pregnant sometimes even having sex during their period

I mark it by the 14 days, not my body temp. I should be starting my period the 17th which is fairly soon. I've had a huge increase in appetite. But I've always been the girl to eat the most. Also, feel like I'm gaining weight. I took a test on Sunday. Could you even tell on one of those early pregnancy test that you were pregnant before you period?