IF the blood test said neg. Then you are not pregnant unless you had unprotected sex again after the neg. Blood test if that is the case then there is a chance of being pregnant. It sounds to me like your just haveing an irregular period it happens sometimes due to stress, diet, hormonal imbalace, the phase of the moon..heck you name it just about anything can cause it. Women who have their periods like clock work robot style are the few and far between lucky ones