Thanks for the replies. Hernia, did go through my mind but no pain, lump that I can speak of. Have no other symptoms and feel fine. I do sit at a desk all day and this sometimes occurs during the day (once and very briefly) but mostly in the evening whilst sitting watching TV (again only once). As I mentioned, when it happens, I can stop it but tensing my abdomen - whether that signifies anything I'm not sure.
Thanks