i still havent started my period

it is march 31,2012 and i haven't had my period since Jan 10,2012 and on Jan 10 i only spotted for that day and half of the 11 and i haven't had my period since and i have took 5 pregnancy test and all of them have said neg. and i was wondering if anybody could give me some advice and let me know what you think it could be i am going to go to the doctor next week but it is bothering me and i dont know what to do...so if anybody knows please let me know