What Causes Missed Period While HPT Is Negative?

i am 28 years old i have two kids i have been having my period since i was 16. my period has always been normall until the last year it has been coming later than usual like every 40 to 45 days instead of 28 to 30. i now havent had my period at all since december and it is now the beginning of march i have takin several pregnancy test and they have all said no i also had a papsmear and it was normal. i have had alot of stress but im still worried. whats wrong with me?



sandy m