Treat it ASAP.
I had it with my first baby, starting at around 7 months. it started out like what you described, but eventually progressed to an unbearable, excruciating burning inside the entire breast that lasted for an hour or more after nursing. It seriously felt like someone had rubbed icy hot on the tissue inside - I've also heard it described like shards of glass. It is the most painful thing I have ever experienced (including childbirth) and happened after EVERY time I nursed.
By the time I tried gentian violet (which you can get at Whole Foods) I think it was too far along to do any good. My doctor prescribed Nistatin (sp) cream which helped at first. Eventually, diflucin was prescribed and that helped but I was never able to totally get rid of it. I alternated between thrush and mastitis and I think the antibiotics for the mastitis aggravated the thrush. I eventually had to stop breastfeeding because I couldnt take the pain any more. It was terrible/
Try the gential violet first - you can get it without going to the doctor. Also try cutting out sweets from your diet and eating yogurt with those little bacteria in it (?). if that doesnt take care of it within a week, go to a doctor. If you flinch at the cost, think about how much you have saved by not buying formula... :)
Good luck and take care of yourself.