R.D.
I expect it will get better over time, but it's reasonable to rule out any possible issues. Just call the dr's office and ask one of the nurses if you should proceed with your mammogram. I would think it will be fine. Might be a bit more painful than usual however.
If it's really just the nipple area that's painful, look for some of those gel pads in the nursing aisle; I thought they were really soothing. I've also heard cabbage leaves really work, but gel pads seem a little more discreet. :)