Scraps right, you should be using your legs to get down under the rope and not bending your back..

Although as it's your first day doing this exercise, you may be using some supporting back muscles to help you balance bent down that you don't use much... Just about everytime you do something new that uses muscles you don't use much, you feel feel sore in odd places..

Once your back feels okay just go at it again and see how it feels.. Rememeber you want to keep perfect stance when your bending down, so use your knees.