Hi Jo,
I had a tender boob for about 5 weeks, with a slight red rash you mentioned which was above and below nipple. My nipple was tender (pins and needle feeling) as well as quite hard on the scar side which probably was due to the tissue healing. I had to have more surgery last week, so back to drawing board with my pain/swelling etc....
Maybe call up your Breast Care Nurse and explain all that is happening, she may have some ideas on how to help. Maybe light touching the area to get the nerve ending fired up again, I was told to do this to help the pain (jabbing pins and needles) in my arm after this surgery which is slowly helping.
Hope it gets better for you soon, it has only been 3 weeks which isn't that long, maybe your body is just taking it's sweet ass time to heal and there is nothing to stress about.... :o)
Take Care.
Rachxx