Symptoms of a breast infection (or mastitis) are similar to flu symptoms: headache, weakness, aches, fever (usually more than 101 degrees F), and chills. One breast will also be very tender and probably streaked with red.
If you suspect that your breast is infected, get in bed, get comfy (take off your bra if it's tight), and breastfeed your baby at least every two hours — starting on the infected breast. If you don't think the breast if fully drained after baby feeds, use a breast pump to further empty your breast. You should also put in a call to your doctor, who will probably prescribe antibiotics. Moist heat can help too, as can extra fluids, vitamin C, and mild painkillers like Tylenol and ibuprofen.
Never ignore a suspected breast infection, and don't try to wean during a breast infection. Failure to empty the affected breast frequently can lead to a breast abscess, which may need to be drained surgically. (Not fun.) Starting antibiotics ASAP can protect your breastfeeding relationship, your milk supply, and the health of your breasts.