Hey Laura,
I usually feel major doms...especially lower back. You might just need to get used to it. I use 8,10,15 and 25 lbs for this workout depending on what the exercise is. I actually do 15 for the shoulder presses which I couldn't even do with P90x before. Go higher in weights next time. I almost never get doms in my upper back...only some kickboxing workouts do it for me.