Why did my period come on 9 days early? Is this normal for an 18 year old?

I can't say one day of exercise could be the cause. It seems unlikely from what I know. However, there can be another cause and it's unknown of it's problematic or not.

Yes, it's true that stress and insomnia can affect it this way, too. Extreme diet...with low caloric intake can do it, too. Suddenly adding a higher level of exercise than what is your norm can even cause you to skip a month, too. with moderation, you should probably go back to regularity soon. However, it is a change that a physician should be aware of and could advise you best.

I'll assume there has been no sexual activity. You don't need to answer this..as it's a private question - I'm just suggesting another possible reason for delay. At age 18 there can be irregularity that can also be quite benign.

Perhaps you should make an appointment with your primary care physician? At the very least you should ask your physician about this question (parents call or you do it).

Don't panic, but see how it goes. This might only last a few days, but keep track of your body. If the flow becomes heavy or there are clots, see the doc right away. It might be just be breakthrough bleed, and resolve itself quickly. If it continues, well just keep notes on what happens daily. I think you will be fine, but I'm not a GYN. Just a mature old broad, so if you have questions you don't want the entire forum to read, feel free to PM me. But don't if it makes you uncomfortable. Good luck, GB