In her knackers???

Have been playing for a while, BUT over the last couple of weeks I have been having a little bother with just one of my lady Sims. She has been loosing/removing and not replacing her clothes?? You'll just randomly see her roaming around undressed? At first I thought it may have been that she didn't put them back on after sleeping, showering etc. I've just tapped on her home, house full of people, the host in her under-crackers ha!!
Am I doing something wrong? Is it a glitch? Has anyone else seen this??