Sore breasts = sign of pregnancy
Missed period = sign of pregnancy
Conclusion: take a test, if it's positive decide what you want to do. If it's negative, wait a week, take another. If it's positive you have a decision to make, if that one is negative too then go to docs find out why you've missed a period and check it's not a double false negative.
Unless you come on your period in that time then obviously you're prob okay.
Edit: sorry that came across a bit dramatic. There are also loads of reasons your nipples could be sore like Anna Spanner has said and plenty of reasons your period could be late (if it is late).