I agree with Nursesooz,it sounds like it could be a blocked duct.
It may even be the beginning of mastitis.
If I was you I would call the doctor and get an appointment cos if it is mastitis you want to nip that in the bud as quickly as possible.
I had mastitis a couple of weeks ago and it started off with my breast feeling hard even and peculiar. This was in the evening,I couldn't see any redness at all but by the time the midnight feed came around I actually cried it hurt so much to feed. I got a doc appointment the following morning and the doc took one look at my breast and diagnosed me with mastitis,by then I had some redness. She prescribed me antibiotics and thankfully as I got it so early was able to carry on feeding off that breast.
The thing with mastitis is it can develop quite quickly if it's not treated. My sis had it with her lo and ended up in hospital for 3 nights with it! Hence why I was so cautious and almost looking for it!
Hope it isn't mastitis for you and that it goes away soon though x