OK. If you are a virgin - no sexual experiences - then you KNOW you are NOT pregnant. There are a lot of things that can cause you to miss a period or even two. There are sympathy/empathy pregnancies where your body thinks you're pregnant when you're really not but this is really rare. Stress will cause you to miss a period. If you've just recently started having your period to begin with then your body may need more time to adjust the regularity of your periods. There could be a health issue that's causing the problem. The best thing to do if you miss your period more than once is to see a doctor to find out what's going on. If you've come really close to having sex but not quite then be sure to tell this to the doctor as some stds can be spread by "almost" having sex or performing sex acts.
Missing one period shouldn't freak you out but it is cause for concern. If you have any symptoms of anything other than a missed period then you need to see a doctor. Symptoms can include needing to pee a lot, burning, itching, pain, fever, nausea, vomiting, and anything that's not normal for you.
If you are sexually active at all - even if you're still a virgin then you need to start seeing a gynecologist every year especially if you're having problems.
BTW, Sammykinz13, it's normal to not have a period once in a while. If it's not normal next month then you may need to see a doctor and get started on birth control pills to control your period. The pill will keep you from getting pregnant (most of the time but it must be taken at the SAME TIME EVERYDAY!) but it also will help to get your period in a normal cycle. After you've been on the pill for a few months then you may be able to come off of it and do fine after that. Check with your local clinic, doctor, or health department if you don't get your period next month or if you start and it lasts a lot longer than normal.
Good luck to all of you. Take care and God Bless you.