Thanks so much for the info!
Excessive exercise can cause your period to be late or you to miss it. If you're too skinny that could be the cause too but since you said you're muscular its most likely just the exercise. If you're worried see your doctor and ask!
Listen, it's going to be alright. A cycle, especially in young teens that are still adjusting to their cycles, can be very irregular. I had my first period, and didn't get another one until 6 months after that! I've done Crossfit before - it is very intense. I'm a runner now, and I would sometimes miss my period for months at a time due to the intense training. Excessive and strenuous exercise can very often cause a delayed or missed period. In your case, missing a period is very common. If you haven't had sex, then you don't need to worry. However, if that does not ease any of your worries, go see a doctor :)