Well, pregnancy tests are pretty accurate these days. Most likely it would give you a positive if you were pregnant. Cheap or not. In the teen years, it is not unusual to be irregular. Lots of things interfere with getting periods including stress, changes in routine, illness, weight gain or loss, fluctuating hormones, a slow thyroid. So, it could be a lot of things. But since you may have had unprotected sex, and you'd like to know, you can always go to a doctor. I know that is probably scary at your age and maybe something you don't want your parents to know about. There are places like clinics like Planned Parenthood that offer services to young women. So, you could get a blood pregnancy test there and then discuss why your period is late if negative. I really don't think you are pregnant though. good luck