What Causes Delay In Periods And Breast Pain?

im alomost three weeks late im 23 years old and i have two kids they are four and five and i really dont rember about early pregnacy. i was suppose to start sept 23rd 2010 and next sat is my next missed peroid. i havent really had much sysptoms but sometimes my breast aches. i have takin three dollar store test about a week ago and they say negitve what should i do?