Is it in my head?

I'm really worried that I might be pregnant because my period is late 4 days and I've only had sex once and it was a month ago it didn't last long and he barely came! The more I think about it the more I feel the early stage symptoms but I've not had the big ones like sore boobs but I've had cramps and vivid dreams I need to pee more than usual! I don't know whether it's just psychological or not because I'd only be 4 weeks pregnant but I'm really worried because I'm only 15 and I can't speak to anyone about it! Is there any possible ways my symptoms are all in my head?