Sounds like it might be thrush - is it in both breasts, continuing pain after a feed for up to an hour, deep shooting pain up inside the breast, have you had any antibiotics (cause thrush), has it come on after 3-4 weeks of pain-free breastfeeding, pale looking nipple, baby might become fretful and have white sheen or spots in mouth (though not always visible!)
If it's all this, get to the doctor and get drops for baby, cream for you and an oral pill. It is hard to get rid of so make sure you finish the course, but the pain should go after a day or two.