Paranoia about being pregnant

I am 15, and I lost my virginity 6 months ago as I felt it was the right thing for me. We practised safe sex but I believed I had fallen pregnant. I have had my period since I was 11 and I know my cycle, it is always on time, it stays for the same time every month and is the same consistency. But this time I was 9 days late. I took a test and it was negative then my period showed up 2 days later. Last week I had sex with my boyfriend for the first time. We have done it several times, all unprotected. He pulled out every time. We are both confident that he didn't cum in me but there is a thought in the back of my mind saying what if? I was already 6 days late when we first did it, I am now 15 days late. I read that worrying about a certain thing or being stressed can prolong your period, is this true? My mum fell pregnant with my brother at 16, am I just being paranoid because it could happen to me? Could my mind be prolonging my period? I have talked to my boyfriend about it and we both understand the circumstances and we are scared. I would not go through with it if I was. I plan on getting a test. I guess what I'm posting this for is advice. To explain my possible paranoia, and ways to abort it the worst does arise. Please.