If you have taken home tests and they have come up negative, its one of two things, your not pregnant, or your too early for it to be detected, which is common with some pregnancy tests. Quite a few tests that say "effective from day of missed period" are lieing, and they dont really show a clear result until about a week after your due. alternatly you could be one of the few people that the home tests just dont seem to work for, thought its unlikely. The hard stomach could be just that (lucky you), and burping, maybe you have had too much softdrink lately? Its worth going to the doctors because your late though, just get a blood test to put your mind a rest. If you start getting really sore boobs, incredibly tired, or feeling queezy/throwing up, then definatly go to a doctor as they are some of the earliest signs of pregnancy.
Hope that helped.