Pregnant or not?

Kat
Typically i have my period like clock work. I have it the same time every month for the same length of time and nothing should of changed it but this month i am 13 days late. I have taken 2 pregnancy test 2 days after i missed my period both say negative. 
 Is it possible for a test to read negative and then end up being positive and pregnant? 
How long should i wait to go to the doctor and have a pregnancy test or take another at home pregnacy test?