M.G.
The first thing I thought of was a yeast infection, aka thrush. However, it certainly could be mastitis as well.
I wanted to second the pp who recommended www.kellymom.com for good info! You should easily be able to clear up whatever the problem is WITHOUT antibiotics, if the problem is one of the above. In fact, antibiotics can contribute to yeast infections!
I'd recommend staying away from an MD, unless you know that they are breastfeeding-friendly. Unfortunately, MDs aren't trained to recognize the myriad of benefits that breastfeeding offers both mama and baby.
Most importantly, KUDOS to you for breastfeeding! Don't give up!!