I happen to have an article on mastitis at hand, and it says that if symptoms have not improved within 12hrs, to see your GP for antibiotics.
Things to try:
drain the breast
apply heat (shower/warm face-cloth/wheat pack) to breast before a feed
massage breast towards nipple
after feeding apply cool packs or cabbage leaves for no longer than 20min
rest!
Vitamin C - day one 3000mg, day two 2000mg, day 3,4 &5 1000mg
take panadol
Hope you're feeling better soon!