Could I be pregnant and had a false negative?

ok well i was suppose to qet my period on jan 30 2010 but i didnt and around midnight on jan 31 i went to the bathroom and when i wiped myself the paper was with pink. And the whole day sunday my period was coming very light the pad was kind of clean and on monday feb 1 it started to come a little more but it feels like if it where the last 2 days and now its da 3rd and its coming more; i have had on jan 11, 13 and the 22 and we have used the pull out method; i did a pregancy test and it came out negative what could it be?