Hcg doubled, then did double, then doubled again??

I’m really not sure what’s going on. I had my betas as follows:

11/9 hcg 3

11/12 hcg 21

11/15 hcg 33

11/19 hcg 121

My levels doubled on the second blood draw but on my 3rd they only went up by 12 and so on my 4th draw I was really expecting a lower number or for them to have dropped but they doubled again. I’m very confused and haven’t heard back from my doctor and won’t here back until the weekends over. Has anyone had a similar situation and it turn out okay? I’d love to hear either outcome.