Hi I had EXACTLY THE SAME THING at week 5ish too. My right breat hurt the most and I have a lump in there which I've a had for a few years (had it check by an oncologist and an ultrasound on my boob, looked like a cyst). Anyway, I got a swelling in the gland under my right armpit as if the soreness in my right breast extended. I was panicking but fortunately the pain and lumpiness in my underarm disappeared after 3-4days. Havent had my first check up yet but I'd suspect its the initial surge in hormones that causes stuff like this. Mention it to the doctor when you go, they're supposed to check your breasts at the start of pregnancy. When I saw a breast cancer onclogist a few years ago I was 24 years old. He told me at the time "the chances of you having breast cancer are the chances of a donkey falling on top of your head when you walk out of this office". However, my mothers co-worker got breast cancer at 30 yrs old, after her second pregnancy - it went undetected at first as they tried to say it was just a blocked milk duct. This id why is so important we are aware how are breats look and feel before, during and after pregnancy. I believe some forms of breast cancer are hormone related and pregnancy may aggravate it, with all the hormones it produces.