What causes late periods but negative pregnancy tests?

Hi! Not sure how to ask a question. But, my period is normally very regular. This month it is not, as I am four days late. I am not stressed or anything like that. The only thing I can think that I have done differently is I recently started working out. However, I have worked out in the past, and it’s never interfered with my cycle. I just got off birth control in April, and everything has been normal since. My husband and I have been trying for a baby since then, but I’ve had three negative tests. I can’t ask my mom about this, and nobody knows, but I need help. Any advice?