Reddit mentions of Cycle Savvy: The Smart Teen's Guide to the Mysteries of Her Body

Tests from the dollar store are also surprisingly good. Hopefully this is nothing more than a scare! If you want more information about what's going on with your body during your cycle and how to know when you're fertile, I recommend this book https://www.amazon.com/dp/B003V1WTEU/ref=dp-kindle-redirect?_encoding=UTF8&btkr=1 This is the stuff I wish I would have known much earlier in life.

Hey, it's gonna be ok! I was 12 when I got mine. All of these symptoms are normal for a period, but also you still might not see any blood for a while and that's ok. Sometimes the symptoms can come before the blood. You've already gotten a lot of good advice here, but I wanted to add that you should experiment with different options for managing your cycle. Pads come in various sizes, so what your mom has might not work for you. When you get your first, it can be a little light so you might just want to use a panty liner, or a smaller pad. It's also possible you could have a heavy flow and your mom's pads could be too small. Once you start bleeding you can get an idea of what you need, but I recommend trying one of the starter packs that come with a few sizes. You can also look into tampons. There are a lot of myths about young girls using tampons but they are perfectly safe at a young age. When I got my first one my mom insisted I use pads at first but I found tampons were better suited to me. You'll also want to experiment with sizes on those.

Also, I read this book when I was a kid and found it really helpful. Feel free to message if you have any other questions, and good luck!