Advice please!

Renee 👦🏼👼🏻👧🏻👼🏻👼🏻👼🏻👶🏻 • Stay at home mom. My family is my life.

According to LMP, I’m 6w5d. Started brown spotting yesterday so today we did ultrasound. There was nothing. No sac, nothing. Last week my hcg was 196. Today, it was 899. Sounds good right? Why did they tell me to be cautiously optimistic? I go back in 2 days for another beta.