Worried

Jgubnies
So I've been on coastguard birth control since the middle of January and my periods were regular. Last month it was 2 day's late. No big deal. This month I expected it to be a few days late. Its now going on 9 days late! I've taken 3 him pregnancy tests and all negative. Dr said it could be stress related but I'm under no significant stress! I was wondering if anybody had the same symptoms and some thing serious happened.