I'm so confused

Katie • I`m almost mid 20s owing my own house with my boyfriend of over 5 years we finally have had our dreams come true. 💖 Isabelle Ivy kirtland 💖
I'm so confused. AF is 4 days late and I have been feeling so nauseous. The most I'm ever late is a day. I took a test and it was a BFN. I really don't know what to do. I've had brown string CM on day AF was due but nothing at all since. 
I've been trying for over a year with my BF and we recently found out that our best friend and her bf are having a baby and they wasn't even trying or been together long. I'm heart broken. What do you all think is the reason I'm so late?