I haven't had my period since july 28, 2012, lasted till august 2nd. i took a pregnancy test this month and it was negative. what should i do?

Can be normal: Many people have 32-35 day cycles so this can be normal. If no period after 60 days+test is neg+ you want to conceive, i would call your doctor for Progesterone to bring on period. If this recurs, you may need hormonal evaluation to check for an imbalance in your hormones. It is healthy to have your period every 90 days if not interested in fertility.

Amenorrhea: I would see your ob/ gyn for an evaluation. A physical exam would be helpful, some blood work such as a thyroid study and pregnancy test could be useful. In the majority of cases there is no pathology or medical problem. This problem in your age group is often treated with the "pill".