The problem could be that you just are not accustomed to the movement of your body on the treadmill and you are cramping up. My body will take some time getting used to new constant movements. I usually give up cardio during the winter then when spring hits and I start cutting I'll get back into it. I get lower back cramps the first few times when I run stairs, but they go away after a few workouts. My hips ache a bit also. Same deal when I ride my bike - I ride a mountain bike and my shoulders / triceps and back cramp up at first, but after a few rides I'm good to go. I think it has to do with being in the same position for extended periods of time and the initial shock of something new along with some extra size from winter bulking. Could also be your form on the treadmill, as others have suggested, does this occur if you were running on the road? Maybe your just tense - Are you doing any stretching prior to cardio?
If it hurts real bad, rather than just being uncomfortable, you should switch to something else.