Puppy Peed While Lying Down?

I have a 3/4 Aussie 1/4 border Collie female puppy. She is 7 months old. She was lying down in our kitchen chewing on a toy when she just suddenly peed. She acted completely fine afterwards. This is the first time she's ever done this. There was a thunder storm coming through. Don't know if she was just scared or if there is something else going on.