Pregnant? or just missed period because of stress?...?

A few weeks ago I had a UTI and I was placed on antibiotics for treatment for a week. My period was supposed to start on the 2nd day of taking them and it didn't. I am sexually active. We do not use condoms nor am I on any type of birth control but he does pull out before ejaculation. I have been experiencing cramping since the day I was supposed to start but no cycle started. While on a trip, I got a sunburn on the core of my body so I applied aloe vera on my stomach to take some of the stinging away and passed out on the floor. Then my boyfriend made me take an at home pregnancy test when I got back home (right after sex but we decided to take it because I was 2 weeks late so not because we wanted to know right away after just doing it... just to make that clear) and it came out positive. We decided that because of the stress I am currently in with my household such as family issues and the stress that my body is trying to recover from my sunburn, could have an effect on my cycle. I know cramping isn't good when pregnant so maybe the positive test could've been wrong? Is it possible to secrete preg hormones when not pregnant and just had sex? I checked google everywhere and I can't really seem to find anything on hormones right after sex. Any answers will be appreciated. I'd just like to know if it is at all possible to be in my position and not be pregnant. I'm not too worried about it, the only thing I'm really scared of is not giving my baby proper nutrition. I'm only 17 so I'm a little concerned about getting to the doctor for it as well. Thanks