^
It very well may be. You need to tell your mom. Don't be afraid. She once had to tell her mom too, so your mom knows how frightning it may be. Find a time when your mom is alone, and in a good mood. Don't decide to tell her after you've just got in an argument with her. She will just get mad that you haven't told her sooner. Take a deep breath, smile, and say "Mom! I am not 100% sure, but I think I have got my first period today!" If your mom isn't around, do what I did. I called my mom. It was somewhat easier to tell her over the phone. You could leave her a note under her pillow, or somewhere else only she will find it.
You have nothing to be nervous about! Periods are just a sign you are finally a woman, and are able to reproduce. It's totally natural!
Good Luck!
-Rachel.