She needs them because she used the damaged Peacock Miraculous

Nathalie Sancoeur knew that using an unstable and damaged Miraculous would be a risky move, but if it meant saving someone she cared for, she was willing to take that chance.

One risk she didn't see coming was the sudden case incontinence that was a side effect of her sickness after first using the damaged Peacock Miraculous. Thankfully, as embarrassing as it was, the person she saved, Gabriel Agreste, was more than willing to pay for the diapers she would now need to wear for the foreseeable future. It was still a lot to get used to, but she tried her best, even if she occasionally stunk up meeting rooms.

One day, Nathalie decided to transform into Mayura once again to see if there were any people she could use to create Sentimonsters with. Much like the rest of her clothing, Nathalie felt her diaper was different, somehow even thicker than usual. She decided to put that aside for now, and started her patrol.