You can wait 24 hours (some even say 48 hours) and watch for the fever to be going down. If it is you can avoid the antibiotics. It's good to do so - b/c you can end up with thrush from the antibiotics for mastitis and then have a whole new set of problems. BED REST, NURSE, warm moist heat on any lump, Ibuprofen. HEre is a fabulous hand out http://www.kellymom.com/bf/concerns/mom/mastitis.html
If you are uncertain you can also call a local La Leche League Leader for ideas and support. If you are not signifigantly better in 48 hours or if you are in coherant (I got there once) do call a doctor and get a script.
If you have any hard spots they MUST be worked out with nursing, massage, warm heat ect. They can turn absess. Antibiotics alone will not cure mastitis you must address the udnerlying cause. 9Change in nursing pattern - usually abruptly, new routine, increased stress, plugged duct ect.)