Turns out I've lost some weight (without actively trying ). This morning all of my school shirts were in the wash except for one which I never wear because it's always been too small for me. So in desperation I put it on and it fits. The only recent change to my lifestyle was changing my Tai Chi class - from one that pretty much only did forms (boring) to another one which does applications and now incorporates contact push hands (which can get pretty rough - they're seriously talking about investing in jigsaw mats).