Just a guess, but the back pain starts (you get tight) while doing your cardio too? Were you active prior to this? You've started out with a pretty long cardio session (I would say 30 minutes max initially). Often back pain results from a muscle imbalance in your core muscles and really, what we're talking here is flabby abs

(hey, speakin' from experience). Instead of the arm weights at this point, you might want to shift that time to doing crunches and other exercises to strengthen your abdominal region to help balance and support your back muscles while you are doing the aerobic work. Personally, I'd add in some stretches too targeted at these key areas.