I cannot understand where this pain in my shoulders came from

My shoulders are killing me today. I couldn't even hit the pool. A deep muscular ache deep in the deltoids, as if I've been lifting weights (and this week I've only made it into the water twice.)

I've a feeling today is my monthly 'crash day'. I get these about once a month, whether I want them or not: a feeling, usually after a burst of bad humour, that I've been overdoing it and need to relax for a day. My planned day of Corporate Finance fell completely by the wayside; instead I've read, snoozed, enjoyed a Guinness down Varsity, all the stuff a second-term MBA shouldn't really be doing. I'm at a total loss for what to do this evening. But it feels good.