Negative test

According to all my apps my DD is around Dec 11th. Im suppose to be 3 weeks and 2 days. It's 6 days before my missed period and I took 2 first response tests. They both came back negative. I work at a hospital and drank about 7 glasses of water during my shift, came straight home and took the tests. Is it possible my urine could've been diluted and the caused a false negative? I just have a hard time believing in not pregnant since I have so many symptoms.