You could be having mastitis or breast engorgement.It can happen to both breast feeding and non-breast feeding mothers. It is a benign condition. Just maintain a clean hygiene to prevent infection.
Frequent breast feeding, different positions to help drain the milk from the breasts could be helpful.You could also try breast massage, milking the breast, warm compresses and wear lose clothes.
If the pain still persists you need to consult your physician to rule out other causes like infection or complications of clogged milk ducts.