What Causes Weight Gain And Vomiting With Negative HPT?

i am concered, because i had my last period on 8/09/2010 and it lasted three days and was very light which is unuasl for me, also i have gained a noticeable amount of weight and for almost 2weeks now i have been vomiting and its only at night. i took a at home test it said negative but the same thing happened to me when i was three mnth pregnant with my son all the at home test said negative. so could i infact be pregnant