I've never had mastitis, but I have had a plugged duct. Is it possible you have this? It can be painful.
I looked in Dr Newman's book, for a definition of mastitis. It says " a bacterial infection of part or all of a breast. It is always associated with a hard, swollen, read, painful area in the breast."
Does that painful area feel hard and swollen? Is it red? If it is just hard, and painful, it is likely a plugged duct. But that can turn into mastitis. If it is still painful to you, you should get antibiotics. In the meantime, try to keep pumping, and see if you can massage that hard area. This can help unplug your ducts, if that's what it is. Dr Newman recommends that if the symptoms are present for more than 24 hours, you should start antibiotics.
I wish I had more advice for you. I just don't have any experience in that area. Keep us posted, and let us know that you've cleared it up!