Perhaps you're still sore from that very fast 500m piece you rowed on Monday. Good to see you on the rowing machine with some consistency. Nicely done!
Well, it depends. I expect it to be hard if I am rowing at, say, under a 2:05 pace. Not so much if I am dawdling along at 2:13 or 2:15. But I think it is hard maybe because I have been doing more upper back work in my gym workouts - heavier deadlifts, bands, pulldowns, etc. I think it is more that than just the one short 500 session of a few days ago. So maybe it will all shake out.
I'd suggest that you try a Feldenkrais practitioner in order to find what's wrong and help things. Anna Johnson Chase in Kent was one of my assistant trainers, and is quite good. There are others in your area.