Lethal phase!!!

How do I find out my lethal phase?
​How do I know when my cervix is high or low? 
​My Brest were sore for two days now no soreness and AF is due on the 21st.
 I've tested everyday. Still a bfn !! 
​AF is due on the 21st. I've tried first response test but still bfn. I'm scared to test again