Daughter pees thru diaper at night?

My daughter is 18months and almost every other night she pees thru her diaper. I make sure she has a completely dry diaper on. Sometimes i will even change it in the middle of the night.Its weird cuz some nights she will go all night & not pee. Then other nights the bed is soaked!! I am tired of changing her bedding at least 4 times a week. She is in Pampers they used to work great. Huggies & luvs give her a rash. Any Ideas? She is starting to use the potty she uses it in the morning when she wakes up & sometimes thru out the day. She also has used it b4 bed but that doesn't make a difference in weather she pees thru or not. Thanks