It also may just be too much weight for your spine.
You can go down in weight until you find a weight that won't cause you pain. In this situation, just dealing with the pain is not a good idea, because it's possible you could hurt your back and make you stop lifting for a week or much longer. The only problem here is that it's strengthening your back but it will be awhile before you're adequately working your calves again.
I suggest that you find some other calf exercise; there's so many: http://exrx.net/Lists/ExList/CalfWt.html
; I just do 'em on the leg press machine.
Your back pain may also be a sign of a lack of strength in your body core. I wouldn't worry about it, but if you have the time and energy you may want to work on that. Deadlifts, squats, weighted side bends... may more possibilities. Be sure to find something you like doing and stick with it.