Picking and eating but don't have the usual issues..

I pick and eat and have done it for as long as I can remember. I didn't even register that I was doing it until I moved in with my boyfriend who noticed and found it totally bizarre. I'm not depressed, don't suffer from anxiety really, I'm not bi-polar. I can't pinpoint when it started, but I have had eczema since childhood. What are the main causes of this? I'm a teacher and am constantly scratching, picking and searching for nits even when I clearly don't have them, but this started long before I was teaching..any thoughts would be gratefully received.