Yes, it could be the beginning of mastitis. Nurse on this breast often and go to see a doctor. With mastitis you will feel like you have a sort of flu, muscle aches, high fever. It usually doesn't last too long, 2 to 3 days and if it is not a heavy case, it can go away even without anitbiotics.
If you go to the doctor and if she gives you an antibiotic, insist that the doctor makes sure it is OK to nurse while on it, or prescribes you another one that is safe. A key to get over mastitis is to nurse or pump from that breast to drain it well. You will need plenty of rest.