Can a period last 1 day that is very lite Also its been going on for the last 3 months. Took a pregnancy test came back negative. What else could be going on?

Yes, it can last one day and be light. Changes in your body may reflect in your period. Did you start Birth Control pills? A diet? under stress? new exercise routine? sick? Any one of these can change a period. Your age can make a difference. If you are young and just starting a monthly cycle it can take as much as a year before it will "settle down" and be truly on time and accurate. If this continues call your doctor just to make sure that there is nothing to worry about.