Similar experience for me. I do aerobics/weights classes 3-4 times a week. But whenever i go away eg to Aus Champs and so miss gym for 7-10 days invariably my first class back legs fine (as one would expect) but dorsal muscles always complain. JUST a week!!!
I haven't used my arms much at all in recent times - been focusing on my legs. Maybe it's time for some upper body work.