At times of stress or upset ,which I'm under now, I do a sort of sucking thing with my mouth. Lips closed, I can't explain it ,even tho i'm doing it now. Wot does it mean ?
People have various concious - and unconcious ways of dealing with stress.
A number of them, including yours, and smoking, nail biting, and so on, relate to the comfort we still derive from putting things in our mouths, which relates directly to the security and comfort of suckling as babies.
As long as it doesn't bother you, I wouldn;t analyse it too deeply.
Andy is right; this is an unconscious reversion to the comfort and security that suckling gave you as a baby. Don't worry about it or you'll just get more stressed. We all have our funny ways, it would be a boring world if we were all the same, wouldn't it.