Haven't had my period in 510 days...?!

sometimes its normal to have one and then not have another for a long long time. its just your hormones getting ready to begin properly, after all its going to happen once a month for a large amount of your life. also if your are taking any birth control that could also be the problem.

This may sound stupid, but was it your actual period? Or did you just have something happen where you had some funny discharge or had a medical condition where you were bleeding and didn't get it checked out? Periods when you first start them can be very irregular but this sounds really weird. Almost like you thought it was it but it wasn't and you haven't had your first period yet. I'd say go and see your doctor and make sure that everything is Ok because irregular bleeding that isn't a period isn't normal and could be something serious. Don't mean to freak you out or anything though.

There are many reasons why you wouldn't be getting periods. From the way you described your physique, you are probably underweight, which is known to lead to irregular periods. This is the most likely cause; however, extreme amounts of stress can also stop or delay periods, so if you are very stressed out, that could also be a factor.

was your last period also your first?

you don't weigh very much for being 'pretty tall', if you are underweight, that could be the problem. But just in case...see a doctor?

start to worry when it gets to 610 days.