Are they too protective?

i was invited to a sleepover at my new (my gramma had only just met her mom)'s house, but the parents were going to be out for 3 hours (from 7-10) and we would be "alone" but they had cells & a guard dog also, we were both 13. My gramma said no.I heard them on the phone(my friend's mom and my gramma) and my gramma said that she said no because I was spontaneous and she was afraid i might make a bad decision. Also, in the past my gramma has made every decision for me and basically treats me like i am 5! Is she overprotective? If she is, how can I tell her w/o hurting her?