5dpo and temp dipped

Rosey • Married 20 years. We have 4 beautiful babies 18, 15,11 and our babyboy blessing after 2 yrs TTC ❤️💙❤️💙 God is Good!
Wth-- I'm 5dpo today and my temp dipped below coverline.. Why? 
This has never happened 
This has never happened