Hcg increasing but not doubling

I am 5+1 by today.

My doctor is concerned about my hcg levels. Here are my numbers:

6/10 (14 dpo ) 78

8/10 ( 16 dpo, 2days from 1st draw ) 122

12/10 ( 20 dpo, 4days from 2nd draw ) 500

16/10 ( 24dpo, 4days from 3rd draw) 1327

Doctor did Transvaginal Ultrasound today and coundn’t see a sac. He was worried about ectopic but till now no sac was detected inside or outside the uterus.

He also requested another hcg test in 48hr and said if the number is not raising as it should be, it better to end this pregnancy next week because it doesn’t look viable.

I have no pain, I am not bleeding. Why doctors keep freaking me out about my hcg level not doubling? I don’t want to end it unless I see a proof or symptoms of ectopic.

Please ladies, I need your experience. I have seen two doctors both of them are not optimistic about my pregnancy.