I am sure the problem is the second one.
I do not have issues while windsurfing, last week I had a session from 9 am to almost 1 pm with some stops; I would say I got 3 full hours in total.
Sunday I had some pain in the lower back, nothing to worry about but bending my back wasn't a pleasure for the entire Sunday.
I am planning to get a couple of lessons next month when I will have few bucks in my wallet, I just got the Ezzy mast this month so pretty out of money for lessons right now.