No, I don't think you're being overly protective of Colleen's arm. You have every right to request an extra set of books for her. I definitely plan to do the same when the time comes. Does Colleen have an IEP or a 504 plan? That's something that you would have written up in one of those documents. If she doesn't already have a plan written up, you might need to meet with the school and get this done.
Recently, I saw a segment about backpacks on a news show. They talked about how bad the backpacks are for children -- all children. They said that the weight of the bag filled should NOT exceed 15% of the child's body weight and definitely not more than 25 lbs. The segment also brought out helpful tips like for the child to go to the locker more often throughout the day in order to carry less, use a rolling bag if the school is one level, and obtaining a 2nd set of books to keep at home.
I'd definitely meet with the school about this. Let us know how it works out for you. -Tina