It could (and probably is) just a skin rash. Has it been growing in surface area, or staying the same size? One thing to do right away is to get out a permanent marker and outline the rash, so you can track it's growth.

It could also be mastitis (an infection of the breast) -- for which you would need antibiotics.

But it could also be Inflammatory Breast Cancer, which is rare, but spreads very quickly, so it needs to be diagnosed right away. Inflammatory Breast Cancer looks like a warm red rash, and there is no lump.

Because of the possibility of inflammatory breast cancer (however rare, it is still a possibility), I think you should go to your doctor right away (if it were me, I would try to go tomorrow if possible). Because many GP's know little about inflammatory breast cancer, I would also print out an information sheet on the disorder and ask your doctor for a mammogram + ultrasound.

My intention is not to alarm you. Again, it's probably just a skin rash, or it could be mastitis (which is an infection of the breast). In fact, I had a similar rash on my right breast 2 years ago and it was nothing. It went away on it's on after a couple months. But because of the tiny but serious risk of inflammatory breast cancer, you do need to take this seriously, just in case. It's just one of those rare things you need to rule out to be safe.

For you own peace of mind though, don't go run off googling inflammatory breast cancer!

BTW, here is an info sheet you can bring in to your GP:
http://ww5.komen.org/uploadedFiles/C...806-03202a.pdf

Please remember, I'm not trying to scare you. I just want to make sure you are informed so you can best advocate for your health. If you have any questions, feel free to PM me, and I'll do my best to answer.