Leaky Diaper!

Help! LO’s been leaking out the back of his diaper for the past 3 nights out the backside and it’s only when he poops and then continues to just lift his legs up and down and up and down. We’ve tried his normal daytime size and we sized up for overnight to try that out but that didn’t work either. We’re using Pampers Pure right now as that has seemed to work the best for us but maybe we need a different brand with a better blowout barrier on the back? Has this happened to anyone else and what have you done to remedy this??