Pregnancy tests are not always 100% accurate.
To be on the safe side do another one of go to the Drs for a blood test.
Have you had unprotected sex?

ok thats not enough info we need more
when did you have unprotected sex? when was your period due?

if the test was neg then your most likely not pregnant. but you can go get a blood test to ease your mind.
however back aches and sore breast do not automatically equal pregnancy. they can be pms symptoms.

I started having sore breasts and thought maybe I could be pregnant. 1 week later I took a pregnancy test and it was negative. I wasn't convinced, so I got a blood test 2 days later. Negative. I still wasn't convinced because I know my body. I week later I took another at home test.... Positive!! I knew it all along and everyone thought I was nuts!!

I am having a similar promblem. My boobs are sore and slight swollen boobs. My husband and I trying to have a child. I am suppose get my period this week, took a pregency test today and it was negative. I have never had sore or swollen boobs before I started my period so could I still be pregnant or just a change in my period?