Hey there. The home pregnancy tests you took which gave you a negative are most likely accurate, considering your period is three and a half weeks late. Some hpts, like first response, are able to pick up on the pregnancy hormone 5 days before your missed period. Typically, a hpt will be accurate the day you miss your period.
I think you should call your doctor and rule out reasons why your period hasn't come yet. It could be stress or an underlying health condition. Good luck to you.