Well I have never used straights but I would think it is possible that there might be some stiffness when you first change. It would seem like you have to hold them differently since they, essentially, just fit into the palm of your hand instead of sort of down your arm. also because the needles are so much more slippery than what you are used to you might be hangin' on tighter too.
Anyway, it seems like you would be using the muscles in your arms slightly differently than what you had been so it would make sense that they might be a bit sore at first.
That being said... I have no idea what I am talking about since I have never used straights. It is very likely that I am full of crap!