Im going to be honest with you, go to your mom aot down and tell her. She is your mother im sure she will understand.
You are human and you made a mistake.
Now repent to Him. He is merciful and loves forgiving.
Just never repeat this.
I once lied of my period too when i was in grade 7 not at home at school. But then i actually got them. I swore never to do it again.
And I get you on the prayer part too.
I feel horrible when i think of it but i didnt feel like praying and didn't pray until i realized what I was doing with my life.
Allhamdullilah its all good now. Juat tell your mom.
I will make dua for you that this is made easy for you.
But lying will only make it worse. Believe me.