negative pregnancy tests?

Just wondering who else has had negative pregnancy tests before a positive? I haven't had a positive yet,  however I'm 2 weeks late and I've never been this late before! The doctor just said some people miss periods sometimes, and I know that. But I never have in over 10 years. My boobs are a little swollen and I feel a little light headed sometimes but can't tell if I'm thinking about that because I think I'm pregnant! I've taken three tests, all negative yet still no period! Gah! I need to know!